Femtosecond Laser Pulse Surface Structuring Methods and Materials Resulting Therefrom

ABSTRACT

Embodiments of the present invention are generally directed to materials processing methods using femtosecond duration laser pulses, and to the altered materials obtained by such methods. The resulting nanostructured (with or without macro- and micro-structuring) materials have a variety of applications, including, for example, aesthetic applications for jewelry or ornamentation; biomedical applications related to biocompatibility; catalysis applications; and modification of, for example, the optical and hydrophilic properties of materials including selective coloring.

RELATED APPLICATION DATA

This patent application is a continuation-in-part of, and claimspriority to, U.S. application Ser. No. 11/862,449 filed on Sep. 27,2007, as well as to U.S. Provisional Application Ser. No. 60/847,916,filed Sep. 29, 2006, the subject matters of which are hereinincorporated by reference in their entireties.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Contract No.CTS-042506 sponsored by the National Science Foundation. The governmentmay have certain rights in the invention.

BACKGROUND

1. Field of the Invention

Embodiments of the invention generally pertain to the field of materialsprocessing and associates processed materials. More particularly,embodiments of the invention are directed to methods for processingmaterials using femtosecond duration laser pulses, applications of suchmethods, and materials and/or material properties resulting from suchmethods. Even more particularly, embodiments of the invention aredirected to methods for altering the surface structure of metalmaterials using femtosecond duration laser pulses, applications of suchmethods, and materials and/or material properties resulting from suchmethods, including blackened and colored metals.

2. Description of Related Art

Although materials may be shaped or otherwise altered in a large varietyof ways including milling, machining, grinding, etc., in recent years,laser-based alteration of materials has become a common method for avariety of materials processing applications. For example, laseralteration of materials by high energy laser pulses has been used toboth create precise hole patterns in metals or metal films as well asfor more subtle material alterations such as texturing of metals ormetal films by the intense heating/melting/vaporization effects of suchhigh energy laser beams.

Reported methods of laser alteration of materials involves the use of‘short-duration’ (i.e., nanosecond (ns) and picosecond (ps)) laserpulses. See, for example, U.S. Pat. Nos. 5,635,089 and 4,972,061. U.S.Pat. No. 6,979,798 describes the use of laser pulses of preferably lessthan 130 femtoseconds (fs) to specifically burn metal links onintegrated circuits. Thus the use of ultrashort (ns) duration laserpulses for laser processing of materials may achieve results that aredifferent than those from longer duration (i.e., ‘short’) laser pulses.The duration of a nanosecond laser pulse is long enough for the pulse tointeract with the material as it is ejected from the surface.Ultra-short duration, femtosecond (fs) laser pulses, by comparison, arenot long enough in duration to interact with the material ejected fromthe surface of the irradiated substance, since the pulse ends longbefore the hydrodynamic expansion of the ejected material. Anotherdifference between different laser pulse timescales is that thelaser-supported combustion and detonation waves that are commonlygenerated in a nanosecond duration laser pulse do not occur in anultra-short fs laser pulse, again offering up the possibility ofmaterials processing effects and resulting material parameters that maybe difficult or impossible to obtain with longer duration laserirradiation.

In light of the above observations, advantageous benefits may beobtained from the use of ultra-short, femtosecond pulses in theprocessing of certain materials and the altered materials or materialcharacteristics resulting from processing with one or more fs laserpulses. Certain advantageous benefits may also be realized by theability to controllably modify optical properties of a metal.

SUMMARY

Embodiments of the invention are directed to methods for processingmaterials using femtosecond duration (i.e., 1-999 fs) laser pulses,applications of such methods, and altered materials and/or materialproperties resulting from such methods. Particularly advantageousaspects of the invention are directed to methods for altering thesurface structure of metal and other materials by application of one ormore femtosecond duration laser pulses, applications of such methods,and altered materials and/or material properties resulting from suchmethods. According to non-limiting aspects, methods for uniformlycoloring, non-uniformly coloring, and blackening a metal or othermaterial, increasing a material's absorptance (up to nearly 100%),increasing a material's surface area, as well as altered materialsexhibiting these characteristics are disclosed.

As used herein, the term “colored metal” will mean a metal having acolor that is due to a femtosecond laser pulse(s)-inducednanostructuring of the surface of the metal, but not due to artificialor applied coloring, painting, dying, etching, polishing, anodizing, orother non-femtosecond laser pulse(s)-induced nanostructuring of thesurface of the metal. The term “color” will have its typical meaning aswell as the practical consideration that color is observable over thevisible light spectrum. In that regard, however, one skilled in the artwill also recognize the relationship between color and thereflectance/absorptance of light by a material, where ‘white’ wouldcorrespond to high reflectivity and ‘black’ to high absorptivity. Asfurther used herein, the term “base metal” will mean the metal prior tofemtosecond laser surface structuring as it is ready to undergofemtosecond laser pulse(s)-induced nanostructuring of the surface of themetal, in which case it may or may not be polished. According to theembodiments of the invention, the absorptance of the base metal willalways be less than the corresponding absorptance of the fslaser-treated material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph that plots the absorptance of a gold (Au) surfaceagainst number of fs laser pulses of varying fluence from a Ti:sapphirelaser having a central wavelength of 0.8 μm and a pulse duration of 60fs, and further shows four regimes for absorptance change: AB, BC, CD,and DE, according to an illustrative embodiment of the invention;

FIGS. 2A, 2B are scanning electron micrograph (SEM) images of a goldsurface (a) before irradiation and (b) after one shot from the laserdescribed in the legend to FIG. 1, showing nanoscale roughness (2B)corresponding to the region AB in FIG. 1, according to an illustrativeembodiment of the invention;

FIGS. 3A, 3B are SEM images of nanoscale surface structural featuresproduced on a gold surface (region BC of FIG. 1) from the laserdescribed in the legend to FIG. 1: (a) nanobranches after two shotablation; (b) spherical nanoparticles after five shot ablation,according to an illustrative embodiment of the invention;

FIGS. 4A, 4B are SEM images showing nanostructure-covered, laser inducedperiodic surface structures (NC-LIPSS) in an irradiated area of a sampleafter 20,000 shots at a fluence of F=0.17 J/cm² (region CD in FIG. 1)from the laser described in the legend to FIG. 1: (a) SEM micrographshowing the period of the NC-LIPSS; (b) nanobranches and supportedspherical nanoparticles in the NC-LIPSS, according to an illustrativeembodiment of the invention;

FIGS. 5A, 5B are SEM images showing (a) NC-LIPSS on the periphery of theirradiated area and gold-black deposit outside the irradiated area after10,000 shots at a fluence of F=1.1 J/cm² (region DE in FIG. 1) from thelaser described in the legend to FIG. 1; (b) gold-black deposit after20,000 shots at F=0.17 J/cm² (region CD in FIG. 1) consisting ofspherical aggregates with a mean diameter that decreases as the distancefrom the crater increases, according to an illustrative embodiment ofthe invention;

FIG. 6 is an SEM image of spherical nanoparticles in a sphericalaggregate of the gold-black deposit shown in FIG. 5;

FIGS. 7A, 7B are SEM images of (A) a crater produced by 5,000 shots atF=0.17 J/cm²; (B) SEM image of a crater produced by 5,000 shots at F=1.1J/cm², according to an illustrative embodiment of the invention;

FIG. 8 is a graph of the residual energy coefficients of aluminum (Al)versus laser fluence following ablation with a single 55 ns pulse of aNd:YAG laser at various ambient gas conditions, presented forillustrative effect;

FIG. 9 is a graph of the residual energy coefficients for aluminumversus laser fluence following ablation with a single 45 ns pulse of aruby laser at various ambient gas conditions, presented for illustrativeeffect;

FIGS. 10A, 10B are open-shutter photographs of plasmas produced by 55 nsNd:YAG laser pulses in 1 atm air and in vacuum at (A) F=4.7 J/cm² and(B) F=19.5 J/cm², where the laser beam is normally incident on thesample from the left (the white dashed lines indicate the front surfaceof the sample), according to an illustrative embodiment of theinvention;

FIG. 11A is a graph of estimates of surface temperatures of Al samplesfor a Nd:YAG laser pulse at F_(abl) approximately equal to F_(pl)=1.4J/cm² in 1 atm air (solid line) and at F_(abl) approximately equal toF_(pl)=2.7 J/cm² in vacuum at a base pressure of 0.01 torr (dottedline); FIG. 11B is a graph of estimated surface temperatures of Alsamples for a ruby laser pulse at F_(abl) approximately equal toF_(pl)=1.1 J/cm² in 1 atm air (solid line) and at F_(abl) approximatelyequal to F_(pl)=2.1 J/cm² in vacuum at a base pressure of 0.01 torr(dotted line), according to an illustrative embodiment of the invention;

FIG. 12 is a graph of the residual energy coefficients of Al in air atvarious pressures versus laser fluence following single pulse fs laserablation using a Ti:sapphire laser producing 60 fs pulses with a centralwavelength of about 0.8 μm at a base vacuum pressure of about 0.01 torr,according to an illustrative embodiment of the invention;

FIG. 13A is a SEM image of a mechanically polished Al surface beforelaser irradiation; FIG. 13B is a SEM image of a typical surfacemodification of the Al after 1 shot at F=F_(abl)=0.053 J/cm² in 1 atmair using the fs laser described in the legend to FIG. 12, according toan illustrative embodiment of the invention;

FIG. 14 is a SEM images of the Al surface after 1 shot at F=F_(pl)=0.086J/cm² in 1 atm air using the fs laser described in the legend to FIG.12, showing the number and size of spherical nanoparticles on thesurface being greater than those at F=F_(abl) (i.e., than in FIG.13(B)), according to an illustrative embodiment of the invention;

FIG. 15 shows open shutter photographs of plasma produced by a single fslaser pulse at F=1.16 J/cm² in vacuum (pressure of about 0.01 torr)using the laser described in the legend to FIG. 12, where the laser beamis normally incident on the target from the left (the white dashed lineindicates the front surface of the sample), according to an illustrativeembodiment of the invention;

FIGS. 16(A-D) are SEM images of nanoscale structures in the center ofthe irradiated spot on a copper sample following ablation at F=0.35J/cm² using a Ti:sapphire laser with a central wavelength of 0.8 μm anda pulse duration of 65 fs: (A) sample surface before irradiation; (B) adifferent area of the copper surface after one shot ablation showingrandom fine nanostructures in the form of nanoprotrusions, nanocavities,and nanorims; (C) after two shot ablation; (D) after 1,000 shotablation, according to an illustrative embodiment of the invention;

FIGS. 17(A-D) show SEM images of the central part of the irradiated spoton copper following ablation at F=1.52 J/cm² using the laser describedin the legend to FIG. 16: (A) surface after one shot exhibiting randomnanostructures in the form of nanoprotrusions and nanocavities; (B)surface after two shot ablation showing random nanostructures in theform of spherical nanoprotrusions and nanocavities; (C) surface after 10shots showing both nano- and microstructures; (D) surface after 1,000shots showing predominantly microstructures, according to anillustrative embodiment of the invention;

FIG. 18 shows a SEM image of copper following two shot ablation at F=9.6J/cm² using the laser described in the legend to FIG. 16, showing onlymicrostructures in the central area and nanostructures on the peripheryof the ablated spot; the insert shows microstructural details in thecentral area;

FIG. 19 shows a summary graphic of the different types of structuralfeatures observed under a SEM on a copper surface as a function of laserfluence and number of shots, derived using the fs duration laser pulsesobtained from the laser described in the legend to FIG. 16, according toan illustrative embodiment of the invention;

FIG. 20A shows an image of a copper sample surface before irradiation;FIG. 20B shows an image of nascent nanostructures formed on copper byablation at F=0.35 J/cm² with a single laser pulse using the laserdescribed in the legend to FIG. 16, according to an illustrativeembodiment of the invention;

FIGS. 21A, 21B are SEM images showing the evolution ofnanostructure-covered, laser induced periodic surface structures(NC-LIPSS) in the central area of the irradiated spot on a platinum (Pt)sample at F=0.16 J/cm² delivered from a Ti:sapphire laser system thatgenerates 65 fs pulses with a central wavelength of 0.8 μm: (A) initialrandom nanoroughness formed after 10 shots (the inset shows a detailedview of the nanoroughness); (B) nanostructure-covered LIPSS after 30shots (the inset shows a detailed view);

FIGS. 22(A-D) show SEM images illustrating the formation of NC-LIPSS inthe peripheral area of the irradiated spot on Pt at F=0.16 J/cm² with100 shots using the laser described in the legend to FIG. 21: (A)general view of ablated spot; (B) the magnified details show that LIPSSdisappears in the central area; (C) nanostructure-covered LIPSS with aperiod of 0.62 μm in the peripheral area; (D) further magnified detailof (C), according to an illustrative embodiment of the invention;

FIG. 23 is a graph showing Atomic Force Microscopy (AFM) measurements ofthe surface profile following mechanical polishing and 10 laser shotsusing the laser described in the legend to FIG. 21, according to anillustrative embodiment of the invention;

FIG. 24 is a graph showing a NC-LIPSS profile measured with AFMfollowing 30 laser shots using the laser described in the legend to FIG.21, according to an illustrative embodiment of the invention;

FIG. 25 is a SEM image showing nanostructure-covered LIPSS with a periodof 0.58 μm in the central area of the irradiated spot on Au after 100shots at a fluence of F=0.16 J/cm² using the laser described in thelegend to FIG. 21, according to an illustrative embodiment of theinvention;

FIGS. 26(A-D) are SEM images of nanoroughness on a titanium (Ti) samplefollowing fs laser treatment at near damage threshold fluence of F=0.067J/cm² using a Ti:sapphire laser system that generates 65 fs pulses witha central wavelength of 0.8 μm: (A) sample surface before irradiation;(B) nanoroughness after two shot laser treatment; (C) after 10 shottreatment; (D) a magnified view of a section in (B) showing fine surfacenanostructures in the forms of nanopores and nanoprotrusions typicallyof spherical shape, according to an illustrative embodiment of theinvention;

FIGS. 27(A-D) show SEM images illustrating the nanotopography of Tifollowing femtosecond laser treatment at F=0.084 J/cm² using the laserdescribed in the legend to FIG. 26: (A) nanoroughness after one shot;(B) nanoroughness after two shots; (C) a magnified view of a section in(A) showing fine details of surface nanoroughness; (D) magnified view ofa section in (B) showing fine details of surface nanoroughness;

FIGS. 28(A-D) show SEM images illustrating fs laser produced periodicsurface patterns on Ti following laser treatment at F=0.067 J/cm² usingthe laser described in the legend to FIG. 26: (A) periodic surfacepattern after 40 shots; (B) periodic surface pattern after 100 shots;(C) periodic surface pattern after 400 shots; (D) a magnified view of asection in (C) showing fine details of the periodic pattern covered withnanostructural features, according to an illustrative embodiment of theinvention;

FIGS. 29(A-D) show SEM images illustrating fs laser produced periodicsurface patterns on Ti following laser treatment at F=0.084 J/Cm² usingthe laser described in the legend to FIG. 26: (A) periodic surfacepattern after 20 shots; (B) periodic surface pattern after 400 shots;(C) periodic surface pattern after 800 shots; (D) a magnified view of asection in (C) showing fine details of the periodic pattern covered withnanostructural features, according to an illustrative embodiment of theinvention;

FIGS. 30(A-D) show SEM images illustrating the surface nano- andmicrotopography of Ti following fs laser treatment at F=0.16 J/cm² usingthe laser described in the legend to FIG. 26: (a) nanoroughness afterone shot; (b) nano- and microroughness after 20 shots; (c) typicalmicroroughness covered with nanostructures after 40 shot treatment; (d)typical columnar microstructure after 200 shot treatment, according toan illustrative embodiment of the invention;

FIG. 31(A-D) show SEM images illustrating the surface topography of Tifollowing fs laser treatment at F=0.35 J/cm² using the laser describedin the legend to FIG. 26: (a) nano and microroughness after one shotlaser treatment; (b) typical random microroughness covered withnanostructures after 40 shot treatment; (c) typical columnarmicrostructures after 100 shot treatment; (d) typical columnarmicrostructures after 200 shot treatment, according to an illustrativeembodiment of the invention;

FIGS. 32(A-D) show SEM images illustrating the surface topography of Tifollowing fs laser treatment at F=0.48 J/cm² using the laser describedin the legend to FIG. 26: (a) microroughness covered with nanoroughnessafter 40 shots; (b) typical microstructures following 70 shot treatment;(c) typical microstructures following 100 shot treatment; (d) a craterwith a diameter of about 350 μm after a 1,500 shot treatment, accordingto an illustrative embodiment of the invention;

FIGS. 33(A-D) show SEM images illustrating the surface topography of Tifollowing fs laser treatment at F=2.9 J/cm² using the laser described inthe legend to FIG. 26: (a) smooth surface with microinhomogeneitiesafter a one shot laser treatment; (b) smooth surface with somenanostructures after two shots; (c) a magnified view of a section in (b)showing surface nanostructures; (d) nanotopography of a smooth surfacefollowing 4 shot treatment with observable spherical nanostructures assmall as about 10 nm, according to an illustrative embodiment of theinvention;

FIG. 34 shows a plot of % reflectance versus wavelength in nm forpolished Al (open circles); “black” Al (black diamonds; see also FIG.35(A)); grayed Al (gray circles; see also FIG. 35(B)); “golden” Al (graysquares; see also FIG. 35(C)); and, Al colored by NC-LIPSS (opensquares; see also FIG. 36), according to an illustrative embodiment ofthe invention;

FIGS. 35(A-C) show photographs of metals processed to have differentoptical properties: (A) black Al; (B) grayed Al with two gray shades;(C) golden Al, according to an illustrative embodiment of the invention;and

FIGS. 36A, 36B show photographs of Al colored by NC-LIPSS, where thecolor of the samples depend upon the viewing angle due to a gratingeffect, according to an illustrative embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Embodiments of the present invention are generally directed tolaser-based materials processing using one or more femtosecond duration(i.e., 1-999.99 fs) laser pulses, and to the altered materials obtainedby such materials processing. As used herein, the term ‘materialsprocessing’ and ‘surface treatment’ refer to altering the surfacestructure or restructuring the surface of the material being processedby creating various nanostructures that may or may not be created incombination with additional micro- and macrostructures. Non-limitingexamples of nanostructured surfaces in accordance with embodiments andaspects of the invention are shown in the figures and will be describedin detail below.

Material alterations obtained in accord with embodiments of the presentinvention may be defined by a variety of experimental methods foranalyzing the alterations obtained (synonymously “the materialsprocessing outcome(s)”); for example, by electron micrographic analysis,by spectroscopic analysis (e.g., absorption of light or otherelectromagnetic energy by the altered surface), and via other techniquesrecognized in the art. Material alterations may also be additionallyand/or separately defined in terms of theoretical modeling ofalterations and the mechanisms by which alterations are generated, e.g.,by post-ablation redeposition of material, by the formation ofnanostructure-covered, laser induced periodic surface structures(NC-LIPSS), and others described herein and known in the art.

In this regard, the term “ablation” is used to refer to materialalterations generally, rather than to any specific process of materialalteration. Specifically, “ablation” is defined as occurring byexperimental observation, i.e., by the onset of surface damage oralteration to the material being processed, where the surface damage oralteration is typically observed by eye or by SEM analysis (see, e.g.,Example 2). Thus the term “ablation” is generic, and is not used torefer to a specific physical process of material alteration, forexample, the specific physical process of vaporization or other form ofremoval of material from a surface, etc.

The structural material alterations described herein below may bedefined more precisely as, e.g., “nanostructures,” “nanoscalestructure,” “nanoscale roughness,” or “nanoroughness” obtained byfemtosecond laser pulse nanostructuring of the material. Otheralterations that may or may not occur in the presence of nanostructuringinclude, without limitation, “microstructures,” “microscale structure,”“microscale roughness,” or “microroughness” obtained by microstructuringeffects obtained by femtosecond laser pulse nanostructuring of thematerial, and “macrostructures,” “macroscale structure,” “macroscaleroughness,” or “macroroughness” such as craters or other featuresobtained by macrostructuring effects obtained by femtosecond laser pulsenanostructuring of the material.

With further regard to nanostructures, terms including but not limitedto “nanobranches,” “nanoparticles,” “nanoprotrusions,” “nanocavities,”“nanorims,” “nanopores,” nanospheres” are used to describe nanoscaledimension alterations having the visual appearances under SEM analysisof branches, particles, protrusions, cavities, spheres, channels, etc.With regard to microstructures, for example, “columnar microstructures”is used to refer to microstructures that appear visually under SEManalysis as columns (see, e.g., FIGS. 30 and 31; FIGS. 3, 6, and 26 showillustrative examples of these different micro- and nanostructures).

Further with regard to the above terms, SEM analysis may be used toestablish quantitative as well as qualitative definitions for macro-,micro-, and nanostructures, and these definitions may be used to definethe materials obtained by the materials processing methods according toembodiments and aspects of the invention.

In some non-limiting aspects of the invention, it may be desirable tocreate essentially a single kind of materials structuring, while inother non-limiting aspects it may be advantageous to create ‘mixed’structuring. In this context the word “dominated” is used herein torefer to a situation where one type of structuring is prevalent, i.e.,where one type of structuring occurs across, e.g. about 80% or more ofthe surface area of the surface produced by the specified materialsprocessing regime. In general, however, when the surface is “dominated”by nanostructures, for example, it will be understood that otherpercentage values are explicitly contemplated; i.e., 70, 71, 72, 73, 74,75 . . . 97, 98, 99% (i.e., counting by 1% intervals) of the surfacearea is of the structure specified.

Although the above visually-based terms are used herein to classifynanostructuring, microstructuring, and macrostructuring effectsaccording to embodiments of the invention, other methods can be used tocategorize these structures. For example, because the absorptance of amaterial is a function of the intrinsic absorptance, A_(INTR), and thesurface roughness, A_(SR), alterations to a material that manifest asalterations in surface roughness may be described by absorptance changesrather than, or in addition to, descriptions of macro-, micro-, ornanostructural changes based on changes in the visual appearance. ThusExample 1 below shows in detail how different femtosecond pulse durationlaser processing regimes alter absorptance, and how these alterations inabsorptance correlate with macro-, micro- and nanostructural changes inthe surface of the material.

Further with regard to absorptance, as discussed in Example 1 andparticularly shown in FIG. 1, the materials processing regimes of theembodiments of the invention are capable of producing alterations tomaterials resulting in extremely high absorptivity; e.g., absorptivityfor gold of close to 100%. Such high absorptivity may have particularutility in, e.g., heat absorption applications (e.g., heat exchange andheat absorption for hot water heating from solar energy, etc.). However,as FIG. 1 shows, other absorptance values may also be obtained. Thus thepresent invention is directed to producing materials having absorptancevalues from 0.01, 0.02, 0.03, 0.04, 0.05, . . . , 1.0 (counting by0.01), where the resulting absorptance of the material is significantlygreater than before processing.

The absorptance values determined in Example 1 are measuredcalorimetrically; however, absorptance may also be measured by othermeans, and specifically by methods that allow absorptance to bedetermined as a function of the wavelength of the light impinging on thesample. Reflectivity may also be measured in addition to, or insubstitution for, absorptance, especially in situations where it isdesirable to produce a material with favorably altered reflectivity.Reflectivity may be measured by any standard method used for suchdeterminations; examples of reflectivity measurements are provided in,for example, U.S. Pat. No. 4,972,061, the contents of which areincorporated herein by reference in their entirety.

Thus in a non-limiting, exemplary aspect, materials processing methodsand resulting altered or treated materials are directed to the field ofjewelry. The surface of virtually any metal or metal alloy such as, butnot limited to, gold, platinum, silver, stainless steel, variousprecious metals, decorative metals, and others may be decorated,initialed, patterned, colored, blackened, or otherwise marked viafemtosecond laser surface structuring so as to have, for example,altered reflectivity ranging from the reflectivity of the unmarked metaldown to essentially 0% reflectivity, depending upon the desiredapplication. In various non-limiting aspects, reflectivity may vary withwavelength (producing different colors) and/or viewing angle.

According to the various embodiments described herein, the materials tobe altered by femtosecond laser surface structuring include mostgenerally all metals and alloys thereof, including, but not limited to,gold, aluminum, copper, platinum, titanium, tungsten, stainless steel,and others. The alteration of semiconductor materials and dielectricsare contemplated. Also contemplated are ceramic, glass, and plasticmaterials.

According to non-limiting aspects, metal materials are intended toinclude metal films (e.g., thin metal layers coated on glass, silicon orother additional underlying layer) and bulk metals. Bulk metals refer tonon-thin films of more than a few hundred nm, particularly more than 1μm, and more particularly to more than 10 μm in thickness. Thus “bulkmetals” refers to metals with the characteristics just recited, whereas“thin films” refers to metals of less than a few hundred nm, includingthe thin films described in the Examples below.

Further with regard to the materials of the present invention, as shownin the Examples and in, e.g., FIGS. 13-14, there is evidence thatvarious of the alterations of the materials obtained by the embodiedmaterials processing methods occur preferentially on surface defects ofthe materials being irradiated by the femtosecond laser pulse(s). Thusin some aspects, highly polished materials will be used (pre-processing)in order to reduce the preferential formation of material alterations atmaterial defects; in other situations, it may be advantageous to leavethe material unpolished, to roughen the material, or even to introduceinhomogeneities or other “defects” into the material in order tofacilitate certain alterations.

Pulse duration is a function of the laser system used. In variousnon-limiting embodiments, the laser system is a Ti:sapphire laser systemgenerating 65 fs duration pulses at a central wavelength of 0.8 μm;however, other laser systems generating different fs pulse durations arealso contemplated. See, e.g., U.S. Pat. No. 6,979,798 and U.S.Publication No. 2006/0207976A1, the contents of which are incorporatedherein by reference in their entireties, for non-limiting descriptionsof other such fs duration laser systems, e.g., a Yb-doped fiber lasersuch as the FPCA uJewel (available from IMRA America, Ann Arbor Mich.).Other such fs duration lasers may include, e.g., dye lasers, Cr:LiSAFlasers, KrF lasers, and others known in the art.

In addition to laser pulse duration, a number of other laser parametersmay be varied in various aspects of the present invention in order toobtain the desired materials processing effects, including but notlimited to: the polarization of the laser beam (typically horizontallypolarized); the diameter of the spot of laser irradiation on the surfaceof the material sample (typically between 100 and 1200 μm); thewavelength of the laser beam; the energy density, F (fluence), of thelaser beam; the number of laser pulses (shots) applied to the materialsample; the extent of overlap between multiple laser pulses (shots)applied to the particular region of the material being processed;whether the shots are applied in vacuum or under higher pressureconditions, and others.

According to various non-limiting exemplary embodiments, the fs laserhas a central wavelength (lambda) of 0.8 μm. However, other wavelengthsin the IR, visible, ultraviolet, infrared, THz frequency, etc., may beadvantageously used.

With regard to laser fluence on the surface of the material to beprocessed, as will be discussed below, contemplated fluences will besufficient to alter the surface structure of the metal as describedherein and will be generally below about 25 J/cm² at the materialsurface; i.e., below about 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15,14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 J/cm². The exact choiceof fluence varies, however, depending upon the desired materialsprocessing effects. Thus for example, the summary graphic of FIG. 19shows that different materials properties may be obtained for fs laserirradiation using different combinations of laser fluence and number oflaser pulses.

Further with regard to fluence, in an exemplary embodiment the choice offluence is expressed by reference to the threshold laser fluence(synonymously, the “ablation threshold” or F_(abl)) required for visiblematerial surface damage under SEM. Thus as described in the Examples,materials processing effects can be calibrated to the ablationthreshold, e.g., the fluence specified to obtain a particular effect maybe given both in absolute terms of J/cm² or, alternatively, may be givenas a percentage of the ablation threshold, i.e., as 1, 2, 3, 4, 5, . . ., 100, 101, 102, 103, 104, 105, . . . 10,000% (counting by ones) ofF_(abl).

With regard to laser pulses, embodiments of the present invention mayuse single- and multi-pulse exposures of materials to obtain desiredmaterials processing effects. Laser “pulse” or synonymously, “shot”,refers to a single laser pulse applied to the sample material using forexample an electromechanical shutter to select a single pulse.Multi-pulse or multi-shot situations involve more than a single shot,e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. (counting by ones) up tothousands, tens-of-thousands, or hundreds-of-thousands of shots. Theexact number of pulses or shots chosen will depend upon the desiredmaterials processing outcome, as shown in for example in the summarygraphic of FIG. 19 and as discussed below.

The extent of overlap between shots in a multi-shot situation may bevaried in order to obtained desired effects, e.g., by specifying that atleast x % of the area of an additional shot or shots overlap with thefirst or previous shot, where x can be 1 to 100% counting by ones (i.e.,1, 2, 3, 4, 5, . . . , 100%). Such variations may be particularlyimportant when, for example, the portion of the material in the centerof the irradiation by the laser pulse or pulses undergoes differentalterations as a result of the centrality of the beam than portions ofthe material at the periphery of the pulse or pulses (see, e.g., FIGS.18 and 22).

As a result of shot overlap or other controllable parameters, a variablepercentage of a surface may be altered to have the desired structure orstructures. For example, a precise scanning pattern of a laser beamacross the surface of the material may be used to ensure that a variablepercentage of the surface is altered to possess the desirednanostructure(s), microstructure(s), macrostructure(s), or combinationsthereof. Contemplated percentages of a surface to be modified range from1 to 100% counting by ones (i.e., 1, 2, 3, 4, 5, . . . , 100%). As shownin FIG. 18, precise patterns of laser irradiation application, either atone fluence alone or in a combination of fluences (e.g., highfluence/low fluence) may influence the type of structuring of thematerial obtained. FIG. 18, for example, shows that a two shot highfluence regime at F=9.6 J/cm² on copper will produce a mixed materialsprocessing result of a microstructured central area surrounded by ananostructured periphery.

In addition to specifying the percentage of the surface to be modified,the materials processing effects may also be expressed in terms of atotal area modified, e.g., 0.1, 0.2, 0.3, 0.4, 0.5, . . . 10,000 cm²(counting by 0.1 cm² units). In this regard, it is advantageous that theembodied materials processing methods produce sufficiently large amountsof altered materials, where these amounts may be specified in terms ofthe total surface area of the material that has been altered.

The exact surface area or range of surface areas required for anyparticular application of the present invention will depend upon theapplication; aesthetic applications such as jewelry, for example, willrequire relatively small amounts of altered material. In contrast,larger surface areas of altered materials may be required for otherapplications, e.g., formation of heat absorptive surfaces, or forapplications for, e.g., catalysis or materials implantation into thehuman body for, e.g., dental implants or other situations wherenanostructuring is advantageous for cellular growth and penetration intothe implant material.

According to non-limiting aspects of the invention, advantageousnanostructured materials processing effects may be obtained underambient air/pressure conditions. Thus the pressure conditions underwhich materials processing occurs affect both the threshold laserfluence (synonymously, the “ablation threshold” or F_(abl)) required forvisible material surface damage under SEM and the plasma ignitionthreshold (F_(pl)) as assayed by the onset of bright violet radiationfrom the laser-irradiated spot as measured either by a photomultiplieror an open-shutter camera (see, e.g., FIG. 12). Therefore, variousembodiments of the present invention are directed to materialsprocessing at: low-pressure conditions (e.g., below 5 torr), where, forexample, related materials processing to produce “gold-black” is done;vacuum conditions (i.e., below 0.1 torr); between 5 torr and 760 torr (1atm), i.e., 5, 6, 7, 8, 9, 10, . . . 760 torr (counting by ones); and,at atmospheric pressure, where the Examples provided below showdesirable materials processing effects can occur, contrary to priorteachings that materials processing must be performed at low pressure.

Additionally, Example 2 below discusses the effects of ambient airversus a highly reactive gas (oxygen) versus an inert gas (helium) onmaterials processing using ns duration pulses, and concludes that theseeffects are dependent upon gas pressure, rather than the type of gasenvironment used. While these effects are expected to be applicable tofs duration pulses as well, non-limiting aspects of the presentinvention nevertheless contemplate the use of purified gases in additionto ambient air for use with the materials processing regimes. Inertgases may have particularly desirable effects, thus such gases or otherpurified gas or mixtures of gases may be used in aspects of the presentinvention.

Exemplary Method Embodiments

Embodiments of the invention use pulsed laser beams of femtosecond (fs)duration to obtain nanostructuring of material surfaces with or withoutmicrostructuring and/or macrostructuring effects. The specificconditions for generating a particular structure (macro-, micro-, ornano-) or combination of structures is a function of a number ofvariables including laser pulse duration, laser energy density orfluence (in J/cm²), and the number of pulses or “shots” of the laserbeam delivered to a particular region of the material to be altered.

As shown in the Examples below, both on theoretical and experimentalgrounds, fs pulses produce different materials processing effects thando ps and ns duration pulses. It is also observed that the materialsprocessing effects obtained with fs laser pulses are not a prioriuniform; rather, they depend in part upon the specific pulse parameterssuch as the energy density or fluence of the laser beam and the numberof pulses of the laser beam applied to the sample. However, variouscombinations of beam fluence and shot number may advantageously producemore uniform materials processing results.

Example 1 below will describe the effects of beam fluence and shotnumber on a gold sample. The results are categorized into four discreteregions of effect, AB, BC, CD, and DE as shown in FIG. 1, which shows agraph of the absorptance of a gold (Au) surface against number of pulsesof varying fluence from a Ti:sapphire laser (central wavelength of 0.8μm; pulse duration of 60 fs). The different absorptance values of thelaser beam irradiated gold in various of these regions can be correlatedwith differences in the materials alterations achieved; i.e., region ABis associated with nanoscale roughness (see also FIG. 2); region BC isassociated with nanoscale roughness including nanobranches (see FIG. 3(a)) and spherical nanoparticles (see FIG. 3( b)) and also containsmicroscale structures such as micropores, circular microgrooves, andcentral microchannels; and, region CD contains macroscale structuressuch as craters, periodic structures, and other surface deformations(see, e.g., FIG. 7).

As Example 3 will show, the regions defined in Example 1 and shown inFIG. 1 are applicable not just to a gold material sample but are alsoobserved to be very consistent across different materials. Thus Example3 is specifically directed to an analysis of the effects of fluence andshot number of a fs laser beam on copper, with the SEM results forvarious experiments shown in FIGS. 16-18 and 20. FIG. 19 is a summarygraphic that shows that there are essentially three regions defined bythe data of Example 3: a region dominated by nanostructures (the Xregion in the figure); a region dominated by microstructures with somenanostructures (the open circle region in the figure); and, a regiondominated by macrostructures with some micro- and nanostructures (thesold squares in the figure). These three regions correspond to regionsAB, BC, and CD (or possibly CD/DE), respectively, of FIG. 1 anddemonstrate that although there would be no a priori ability to predictthe existence of these regions, once the regions have been defined, thestructures formed for each region are relatively predictable.

Further confirmation of the general applicability of the three regionsof FIG. 19 to other materials including, but not limited to, othermetals, semiconductors, and dielectrics is provided by the data ofExample 5, where titanium metal was exposed to a varying number of fsduration laser pulses of varying fluence. Specifically, Example 5 willshow that nanostructures are present with low laser fluences (see, e.g.,FIGS. 26-29) as expected, and that for higher fluences of, e.g., 0.16 or0.35 J/cm² and a sufficient number of laser shots (e.g., 20-200; seeFIGS. 30 and 31), microstructuring occurs as predicted by the data ofFIG. 19. Finally, again as predicted by the data of FIG. 19, for 1,500shots at a fluence of 0.48 J/cm², macrostructures are formed aspredicted (see, e.g., FIG. 32( d)).

Exemplary Applications

The apparatus and method embodiments described herein may have utilityin a variety of applications including, but not limited to: aesthetic ormarking applications such as the application of patterning or coloringto the surface of jewelry; medical applications, e.g., for implantablemedical devices, where the novel properties of the laser altered surfaceof such a device may aide in, for example, integration of cells of asubject into the implant; catalysis, where the properties of the alteredmaterials and particularly the increased surface area of the materialsresulting from, e.g., nanostructuring, may improve the ability of thematerial to catalyze chemical reactions; heat transfer situations, wherealterations resulting in increased absorptivity may improve, e.g., theefficiency of solar cells and heat sinks; sensor sensitivity, where theunique alterations to materials described herein may be used in both asensor's absorbing element to increase the amount of electromagneticradiation absorbed and also in the shielding around the sensor orsensors to protect them from various forms of stray electromagneticradiation, thereby helping to improve their signal-to-noise ratios; and,stealth technologies or other technologies where the absorption ofelectromagnetic energy such as ultraviolet, visible, infrared, terahertzradiation, etc., may cloak, conceal, or otherwise obscure the objectcoated or shielded with the altered material having the desiredabsorptive properties.

Exemplary Aesthetic or Marking Applications

A non-limiting aspect of the invention is directed to methods formaterials processing that produce altered materials for aesthetic ormarking applications, for example jewelry or other applications wherethe nano-, macro-, or micro-structuring of a material's surface mayproduces desired effects.

The data of Example 1 will show that fs-based laser processing may beused to increase the absorptivity of a material, which can be observedvisually as darkening or blackening of the surface region of thematerial so altered. Accordingly, an embodiment the invention isdirected to a method of obtaining the desired markings. A relatedembodiment is directed to the materials obtained by such processingmethods.

Exemplary Biomedical Applications

A non-limiting aspect the invention is directed to a method formaterials processing that produce materials advantageously suitable forbiomedical applications, particularly medical applications where a metalor metal-clad device is to be implanted into a subject, and alterationsto the metal or metal cladding may act to improve the biocompatibilityof the metal or metal cladding.

In this aspect, suitable materials may include metals, ceramics,composites, and others, that are nanostructured and/or possiblymicrostructured and then introduced or implanted into a biologicalmilieu such as in bone, in tissue, etc., where biocompatibility isimportant for successful introduction or implantation. The materialscontemplated include any as are known for introduction or implantationinto the body, and include, but are not limited to, metals such astitanium, gold, silver, etc., alloys of these metals, composites, etc.The “biological milieu” may include bone, tissue, etc., of a wholeorganism, or of an isolated component of an organism, e.g., of anisolated organ, teeth, bones, etc. Organisms contemplated includeanimals, and particularly mammals, including humans.

Example 5 below will discuss alterations to titanium metal using a fslaser method described herein to alter the surface topography oftitanium for better biocompatibility, i.e., to provide a surfacecontaining, e.g., pits, pillars, steps, etc., or other structuralfeatures that serve as anchors or other attachment, scaffolding, orstimuli for protein and/or cellular integration.

“Biocompatibility” as used herein refers generally to alterations in thesurface of a material that increase the ability of that material tointegrate into the body, e.g., increase structural integration such asby invasion or interpenetration of the material by cells of the body orproteins or other biological material. Biocompatibility also refers toalterations that increase integration by decreasing rejection of thematerial by the body, as would occur if the material fails to integrate,i.e., so that the body recognizes the material as non-integrated andthus acts to encapsidate or otherwise reject it.

Biocompatibility may be assayed in a variety of ways. For implants, forexample, biocompatibility may be determined by assaying the mechanicalstrength or stability of the integration of the implant into the body.Thus for example, in osseointegration of dental implants,biocompatibility may be assayed by determining the force required todisplace or separate out the implant from the surrounding bone.Biocompatibility may also be determined by directly observing (e.g., bySEM) the extent to which proteins, cells, or other biological materialsare able to invade or integrate into the metal or other material alteredby the materials processing methods of the present invention. As anothernon-limiting example of an assay for biocompatibility, methods formeasuring cell death or proliferation may be used to determine theextent to which the altered surface topography of the material processedby the laser methods of the present invention results in the activationof cells to proliferate, or the active suppression of cell deathmechanisms that would otherwise occur if the cells failed to findthemselves in a suitable proliferative environment.

Exemplary Catalysis Applications

A non-limiting aspect is directed to a method for fs laser pulsematerials processing that can produce materials with desirable catalyticproperties; i.e., materials that contain nanostructural and macro-and/or micro-structured alterations that increase catalytic surfacearea.

Such alterations may be assayed by SEM or other analyses that allow forthe determination of the porosity or other increased surface areaaspects of the materials altered. Alternatively, catalytic activity maybe measured directly by determining the rate at which a reaction iscatalyzed by an unaltered material (e.g., platinum) versus the rate ofthe reaction using an altered material.

Exemplary Modifications of the Optical Properties of Materials

An aspect of the invention is directed to a method for altering theoptical properties of materials, including, but not limited to, metalssuch as are provided in Example 6 below. Thus as shown in Example 6, thematerials processing methods of the present invention may be used toobtain, e.g., metals which appear to the human observer to have variousshades of gray (where “gray” may alternatively be defined as a materialhaving relatively uniform reflectance across the entire visiblewavelength), including multiple shades of gray in one metal piece. Thesematerials processing methods may additionally be used to obtain whatappear to the human observer to be colored materials (where “colored”may alternatively be defined as a material having preferentialreflectance in some regions of the visible spectrum and not in others),e.g., colored metals such as are also described in Example 6, and‘black’ metal. Although these methods are applied to metals in Example6, the present invention explicitly contemplates the application ofthese methods for certain non-metal materials as well.

Example 1

Experiments in support of embodiments of the invention have demonstratedthat a significant amount of residual thermal energy is deposited inmetal samples following multi-shot femtosecond laser ablation.Traditionally, it was commonly believed that one of the most importantadvantages of femtosecond laser ablation is that the energy deposited byultrashort laser pulses does not have enough time to move into the bulksample; therefore, the residual thermal energy remaining in the bulksample should be negligible. In contrast to this, a significantenhancement in laser light absorption was observed recently by theinventors following ablation. To understand the physical mechanisms oflaser energy absorption, the change in absorptance of gold due tostructural modifications following multi-shot femtosecond laser ablationwas directly measured. The measured data indicates that there is asignificant absorption enhancement due to nanostructuring in addition tothe known mechanisms of absorption increase via micro- andmacro-structuring. Moreover, nanostructuring alone may enhance theabsorptance by a factor of about three. The physical mechanism of thetotal enhanced absorption is due to a combined effect of nano-, micro-,and macro-structural surface modifications induced by femtosecond laserablation. At a sufficiently high fluence and with a large number ofapplied pulses, the absorptance of gold surface may reach an absorptancevalue of nearly 100%.

The absorptance A of a pure metal with a clean surface consists of twocomponents A=A_(INTR)+A_(SR), where A_(INTR) is the intrinsicabsorptance and A_(SR) is the contribution due to surface roughness. Foran optically smooth metal surface, A_(SR) is about 1-2% of A_(INTR), butthe role of A_(SR) enhances as the surface roughness increases. Formulti-pulse ablation, only the first femtosecond laser pulse interactswith an undamaged surface, since the laser-induced surface structuralmodification develops long after the ultrashort pulse. In this case, Ais governed by A_(INTR), which can be a function of laser fluence due tolaser-induced change in the dielectric constant of the material. All ofthe subsequent laser pulses interact with a structurally modifiedsurface and their absorption is determined by both A_(INTR) and A_(SR).The absorption of a single femtosecond laser pulse by an undamaged metalsurface is dominated by A_(INTR). However, the coupling of laser energyto a metal in multi-pulse femtosecond laser ablation has not yet beeninvestigated, where A_(SR) may have a significant value due to surfacestructural modification.

The instant non-limiting example discusses the effect of surfacestructural modifications on the absorptance of gold in multi-pulsefemtosecond laser ablation when an originally plane and smooth surfacetransforms into a blind hole. This effect is investigated as a functionof the number of applied ablation pulses at various fluences. A reportedcalorimetry technique allows a direct measurement of laser energyabsorbed by the sample. Our data indicate that femtosecond laser-inducedsurface modification enhances the sample absorptance, which can reach avalue close to 100% at a sufficiently high fluence with a large enoughnumber of applied pulses. Scanning electron microscope (SEM) studiesshow that there is absorption enhancement due to nanostructuring, which,alone was seen to enhance the absorptance by a factor of about three.

Experimentally, an amplified Ti:sapphire laser system was used togenerate 60-fs pulses of about 1.5 mJ/pulse at 1 kHz repetition rate ata central wavelength of 800 nm. The laser beam was focused onto amechanically polished sample surface with a 40-cm-focal-length lens atnormal incidence. An electromechanical shutter was used to select thenumber of pulses, N, applied to the sample. The absorptance of theablated spot was studied. After ablation of the sample with a chosennumber of pulses, we reduced the laser fluence to a level below theablation threshold. Subsequently, we irradiated the ablated spot againusing a train of low-fluence laser pulses that would not induce anyfurther surface modification. A certain amount of energy from thislow-fluence pulse train, E_(A), is absorbed in the skin layer of thesample, dissipates via heat conduction in the sample, and causes a bulktemperature rise ΔT. We measured this temperature rise with athermocouple battery that allows E_(A) to be determined calorimetricallyas E_(A)=CΔT, where C is the known heat capacity of the sample. Tomeasure energy E_(I) incident upon the sample, a certain fraction ofincident pulse train energy was split off by a beam splitter andmeasured with a joulemeter. Having measured E_(I) and E_(A), theabsorptance of the ablated spot could be found as A=E_(A)/E_(I).Laser-induced surface modifications were then studied using a SEM and anoptical microscope.

The optical properties of surface modifications were studied followingmulti-pulse ablation at single-pulse laser fluences of F=1.1, 0.35,0.17, and 0.078 J/cm² in air. The ablation threshold F_(abl) for apristine surface was found to be F_(abl)=0.067 and 0.048 J/cm² forsingle-pulse and 500-pulse train irradiation, respectively. The numbersof pulses required to perforate a 1 mm-thick sample at the center of theirradiated spot were determined to be 16,100, 25,000, and 77,000 pulsesat F=1.1, 0.35, and 0.17 J/cm², respectively. This corresponds toaverage ablation rates of 63, 40, and 13 nm/pulse, indicating that asingle laser pulse produces a nanoscale modification in depth. Plots ofabsorptance versus the number of ablation shots, N, at different F areshown in FIG. 1. For an undamaged surface, the absorptance remains aconstant value of 0.12 when measured at F=0.0043 J/cm², which is anorder of magnitude below F_(abl). The absorptance of a structurallymodified surface is significantly greater than that of the undamagedsurface and shows dependence on the number of applied ablation pulses,N.

The A(N) curves for the ablated surface can be characterized intodistinct regions marked with A, B, C, D, and E on A(N) in the case ofF=0.17 J/cm² in FIG. 1. The first of these four regions is region AB,where the absorptance initially increases from 0.12 (undamaged surface)to a value in the range of 0.25-0.33. Typically, this region covers thefirst 1-10 shots. For example, this initial enhancement of absorptancecan be produced by four pulses at F=0.17 J/cm² or by one pulse at F=0.35and 1.1 J/cm². Optical microscopy showed that the irradiated spot wasentirely covered with surface modification following ablation by onlyone pulse when F≧0.35 J/cm², but four pulses at F=0.17 J/cm². Therefore,the enhancement of A with N at F=0.17 J/cm² appears due to both thesurface modification and an increase in size of the modified area frompoint A to B.

In the second region, BC, the absorptance undergoes a slight decrease asN increases. Typically, this region covers approximately the next100-300 pulses. Both regions AB and BC extend to a larger number ofpulses when the surface is modified at F only slightly above F_(abl), asseen from the curve at F=0.078 J/cm² in FIG. 1.

The third region, CD, is characterized by a further enhancement ofabsorptance with the increase of N. This region extends to N on theorder of 10,000 pulses.

The fourth region, DE, was where absorptance reached a maximum valuethat did not change with further increase of N.

Reference is now made to the SEM pictures of surface morphology shown inFIGS. 2-6. In regions AB, BC, and CD, where absorptance exhibitsdependence on N, the following surface modifications were observed: Forregion AB, a characteristic modification is nanoscale roughness (FIG.2). In region BC, two major features were observed. First, nanoscaleroughness develops further in the form of nanobranches (FIG. 3 (a)) andspherical nanoparticles (FIG. 3 (b)). Secondly, microscale structuresbegin to develop in the forms of micropores, circular microgrooves, andcentral micro-channels. In region CD, features resembling a crater witha deep central micro-channel, periodic structures with orientation inthe direction perpendicular to the laser light polarization and with aperiod roughly equal to the laser wavelength (FIG. 4( a)), and a visibleblack halo around the crater were observed. All these laser-inducedsurface modifications can affect the absorptance in various ways. Forexample, surface roughness can enhance the absorption of light both bymultiple reflections in micro-cavities and by variation in the angle ofincidence (angular dependence of Fresnel absorption). Nanoscalestructural features can affect absorptance since the optical propertiesof a nanostructured material can be quite different from the bulk.Laser-induced periodic surface structures (LIPSS) may enhance absorptionof laser energy via generation of surface electromagnetic waves. Inaccordance with non-limiting aspects of the instant invention, theobserved LIPSS, referred to herein as nano-structure-coveredlaser-induced periodic surface structures (NC-LIPSS) havingnon-conventional, finer nanoscale structural features, are shown in FIG.4( b).

The absorption of laser energy in femtosecond laser ablation may also bealtered through re-deposition of ablated material. Examination of theblack halo produced around the crater shows that its elementalcomposition as determined by energy dispersive X-ray analysis isidentical to that for a pristine surface; i.e., the black halo is alayer of the ablated and re-deposited gold. SEM images in FIGS. 5 and 6demonstrate that the black halo has a structure of sphericalnanoparticle aggregates that is typically seen in gold-black films and,which, have been known for their high absorptance in the infrared.Therefore, the gold-black halo can enhance the absorption oflow-intensity wings of the incident Gaussian beam and contribute toresidual heating of the sample. Since re-deposition of ablated materialoccurs both outside and within the ablated spot, the re-deposition ofthe nanoparticles produced by ablation can also enhance the absorptionof light in the ablated area. For example, an enhanced absorption oflight by a semiconductor coated with Au nanoparticles has recently beenreported. Therefore, in femtosecond laser ablation, the enhancedabsorption can occur due to surface nano-, micro-, macro-structures andre-deposition of nanoparticles depending on ablation conditions. Thecombined effect of these surface modifications can lead to virtually100% absorption of laser light in multi-pulse ablation with asufficiently large number of pulses at high fluence as shown in FIG. 1.Almost all of incident laser energy is retained in the sample asresidual thermal energy. This suggests that the energy carried away bythe ablated material is small in Au, and the enhanced absorptanceappears to be the dominant factor in the enhanced residual thermalenergy deposition in multi-pulse femtosecond laser ablation at largenumbers of applied pulses.

Since different surface modifications are superimposed on each other, itis difficult to completely isolate and determine each individualcontribution to the enhanced absorptance. Therefore, we provide thefollowing estimations on the contributions of nano-, micro-, andmacro-structures induced by femtosecond laser ablation. Since surfacenano-structures are the dominant feature in region AB and part of regionBC for N<50-100, and the absorptance increases from 0.12 to 0.25-0.33over these regions (see FIG. 1), nano-structures alone are believed toaccount for the additional absorptance increase of about 0.1-0.2. Thecontribution of two microscale structures, LIPSS and random roughness,is estimated as follows. To estimate the contribution of LIPSS, weablated a sample using p-polarized light and measured the low-fluenceabsorptance A(N) of the ablated spot with both p- and s-polarizations.The curves A(N) of different polarizations were identical, indicatingthat the grating effects of microscale LIPSS on the absorption of laserlight by gold is negligible. To estimate the contribution of microscalerandom roughness, we abraded a mechanically polished sample surface withsandpaper to produce a rms roughness of 3 μm, which is estimated to becomparable to the laser-induced roughness for 100<N<1000. Theabsorptance of this abraded surface was then measured to be about 0.24as opposed to 0.12 for a mechanically polished surface. This indirectlyshows that the random micro-roughness accounts for the additionalabsorptance increase of about 0.12. Macro-structures come into play intwo major forms, deep central channel and concentric ring grooves, whenthe number of pulses is between about 500-1000 and laser fluence ishigher than 0.17 J/cm². Two SEM pictures showing typical macro-structurecraters are presented in FIG. 7. The macro-scale crater formation startsin region CD and, therefore, we believe the progressive increase ofmacro-structure size largely accounts for the absorptance increase from0.4 to about 1.0. However, nano- and micro-structures also developfurther in regions CD and DE and may also contribute to absorptanceincrease to some extent.

Besides the physical mechanisms of enhanced absorption, we also make thefollowing observations about femtosecond laser-matter interactions:First, laser-induced nanostructures alone can enhance the absorptance ofAu by a factor of about three following only 1-3 pulses. This resultsuggests a new direction for future study of optical properties ofnanostructures imprinted on a metal surface. Secondly, we produced a newtype of microscale periodic structure with much finer nanoscalestructures (NC-LIPSS) following ablation with a large number of appliedpulses. Thirdly, re-deposition of laser-induced nanoparticles is seenoutside of the ablated spot leading to the formation of a nanostructuredmaterial known as gold black. Finally, we identified potential newapplications of femtosecond laser ablation for modifying opticalproperties of metals and producing technologically valuable surfacecoatings such as, but not limited to, gold-black films.

Example 2

In this Example, a comparative study of residual thermal effects inaluminum following fs laser ablation was performed. At laser fluencesabove the ablation threshold where plasmas are produced and at asufficiently high ambient gas pressure, an enhanced coupling of pulsedlaser energy into the sample occurs. Furthermore, in contrast to theconventional understanding that residual thermal energy is negligible infs-laser ablation, up to 70% of the incident pulse energy can beretained in the sample following single-pulse fs-laser ablation in 1-atmair. The major factors influencing thermal energy coupling to the sampleare the laser fluence and ambient gas pressure. Residual thermal energydeposition decreases with reducing ambient gas pressure.

Laser ablation using femtosecond (fs) laser pulses has numerousapplications in the field of materials processing and machining and,nanotechnology. Comparative studies have demonstrated that femtosecondlaser ablation has advantages over nanosecond ablation in aspects ofhigher precision, reduced heat-affected zone, and smaller amount ofdebris around the ablated spot. Following laser ablation, a fraction ofabsorbed laser energy is retained in the heat-affected zone, dissipatesinto the bulk of the sample and remains inside as residual thermalenergy that induces the bulk temperature of the sample to rise. This issometimes referred to as the thermal load and is often undesirable inlaser micro- and nano-machining.

The coupling of thermal energy into metals has been previously studiedfor microsecond and nanosecond laser ablation. An enhanced residualthermal energy coupling to metals has been observed when laser fluenceis above a certain threshold value. It has been suggested that, inaddition to the direct absorption of laser light, energy transfer fromlaser-produced plasma can contribute to residual heating. However,mechanisms responsible for thermal coupling are still not fullyunderstood.

We have observed an enhanced residual heating of metals followingmulti-pulse femtosecond laser ablation, where laser-induced surfacemodification has been found to play a role in enhanced residual heatingbut, where this alone could not fully account for the observed amount ofdeposited thermal energy. To exclude the effect of surface modificationon residual thermal response, we investigated single-pulse fs ablation.We also studied the residual thermal response of aluminum (Al) followingns-laser ablation to compare with the results for fs-laser ablation. Weused a calorimetric technique to study effects of laser pulse duration,ambient gas pressure, and laser wavelength on residual heating of Al. Tocharacterize the residual thermal response, we defined a so-calledresidual energy coefficient (REC) K=E_(R)/E_(I), where E_(R) is theresidual thermal energy remaining in the sample following ablation andE_(I) is the incident laser pulse energy. By definition, REC is equal toabsorptance of the sample material when laser fluence is below theablation threshold. Enhanced residual heating occurred following bothsingle-pulse ns- and single-pulse fs-laser ablation in ambient gas at asufficiently high pressure. The major factors governing the residualheating are laser fluence and ambient gas pressure. There is afundamental difference between multi-pulse versus single-pulse ablationbecause multi-pulse ablation may induce absorptance change due toaccumulated surface modifications from multiple laser shots. Thisaccumulated effect does not occur in single-pulse ablation.

Both ns and fs duration pulse effects were examined. The following threelaser systems were used: 1) a ruby laser producing 45-ns pulses (FWHM)at wavelength λ=0.69 μm with pulse energy of 0.6 J; 2) a Nd:YAG lasergenerating 55-ns pulses at λ=1.06 μm with pulse energy of 1.4 J; and, 3)a Ti:sapphire laser producing 60-fs pulses at λ=0.8 μm with pulse energyof 1.5 mJ. Using each laser system, the laser beam was focused onto anAl sample at normal incidence. A fraction of the incident pulse energyE_(I) was split off using a beamsplitter and measured with a joulemeterto allow E_(I) to be determined.

The residual energy E_(R) that remains in the sample following ablationcauses the bulk temperature of the sample to rise by ΔT. Using athermocouple attached to the Al sample, ΔT was measured after thermalequilibrium was reached in the sample. Knowing the specific heatcapacity c_(p) and the mass m of the sample, the residual energy can beobtained from E_(R)=mc_(p)ΔT. The thermocouple response time (the timerequired for achieving a maximum thermocouple signal in our calorimeter)was about 2.5 sec. Using measured E_(I) and E_(R), the residual thermalenergy coefficient K=E_(R)/E_(I) can be found as a function ofsingle-pulse laser fluence F=E_(I)/S, where S is the laser beam area onthe sample. The samples were mechanically polished. Measurements wereperformed in various ambient gases and at different pressures. Thesample was translated with an X-Y stage so each subsequent laser pulsewas incident on a fresh spot. Two parameters, the ablation thresholdF_(abl) and the plasma ignition threshold F_(pl), were determined at theonset of surface damage visible to eye with subsequent examination undera scanning electron microscope (SEM). F_(pl) was determined by observingthe onset of bright violet radiation from the irradiated spot usingeither a photomultiplier (PMT) or an open-shutter camera, both properlyfiltered to cut off scattered laser light.

Although embodiments of the invention are directed to fs-duration laserpulses, this Example probes some of the effects of both fs- andns-duration laser pulses. The dependence of REC on laser fluence Ffollowing single-pulse ns-laser ablation in various ambient gases underdifferent pressures on Al are plotted in FIG. 8 (for Nd:YAG laser) and 9(for ruby laser). For the Nd:YAG laser, ablation and plasma ignitionthresholds in 1-atm air are determined to be F_(abl)=1.2±0.3 J/cm² andF_(pl)=1.4±0.4 J/cm². For the ruby laser, these values areF_(abl)=1.0+0.2 J/cm² and F_(pl)=1.1±0.3 J/cm². Thus F_(abl)≈F_(pl) inthese experiments. By definition, REC should be equal to the absorptanceof the material when it is irradiated by low-fluence laser light thatdoes not cause any surface modification. The measured value of REC(K=0.25) at F<F_(abl) in FIG. 2 agrees with the reported value ofabsorptance for a mechanically polished Al sample at λ=1.06 μm (D. E.Gray (Ed.): American Institute of Physics Handbook, 3rd edn.(McGraw-Hill, New York, 1972)), and this agreement shows the accuracy ofour measurement technique. Data at 1-atm in FIGS. 8 and 9 show that RECenhances abruptly at a certain fluence threshold, F_(enh), and reaches amaximum value of about 0.5-0.6 indicating that about 50-60% of the laserpulse energy can be retained in Al following nanosecond laser ablation.Our experiment also shows that F_(enh)≈F_(pl) within the experimentaluncertainty for both Nd:YAG and ruby laser ablation.

Next, we studied the pressure effect on REC; representative curves areplotted in FIGS. 8 and 9. For Nd:YAG laser ablation, REC slightlydecreases when air pressure, P, decreases from 1 atm to about 30 torr,but REC abruptly drops when pressure further reduces from 30 torr toabout 0.6 torr. For P<0.6 torr, the onset of plasma is accompanied witha drop of REC. This drop becomes more pronounced as the pressure isfurther reduced to 0.04 torr. At this pressure, REC eventually reaches avalue of about 0.12 that is smaller than the absorptance of an undamagedsurface by a factor of two. For P<0.04 torr, REC virtually remainsindependent of the residual air pressure. This behavior shows that, incontrast to the observation in air, the onset of plasma in vacuum isaccompanied by a drop of REC. In vacuum, both F_(abl) and F_(pl) arehigher than those at 1-atm air pressure by approximately a factor oftwo. Dependence of REC on laser fluence is also studied in 1-atm oxygenand 1-atm helium, and these REC data are shown in FIGS. 8 and 9. Thedependences show virtually the same behavior as those in air, indicatingthat REC does not essentially depend on the particular type of gas. Thecontribution of possible exothermic chemical reactions that may occurdue to presence of chemically active gases such as oxygen is negligible.

In vacuum, the laser plasma mainly consists of ionized species ofejected material, while in a gas medium, plasma consists of ionizedspecies of both ablated material and ambient gas. A characteristicfeature of ambient gas plasma produced by ns pulses is that the plasmaexpands due to the generation of laser-supported absorption waves. FIG.10 shows open-shutter photographs of plasmas produced by 55-ns Nd:YAGlaser pulses for ablation of Al in both air and vacuum under the sameexperimental conditions. Distinction between plasmas can be clearlyseen. The size of plasma in air is larger than that in vacuum.Therefore, the role of plasmas in residual heating of the sample in airmay differ from that in vacuum.

The direct absorption of laser energy is a factor that may influenceresidual heating. According to the Drude model, when the temperatureincreases material absorptivity should also increase due to an enhancedcollision frequency between free electrons and thermally vibratinglattice atoms. Therefore, one should expect an increase in REC withlaser fluence due to this enhancement of material absorptivity. However,the fact that REC increases in air while it decreases in vacuum above acertain laser fluence indicates that the temperature-enhanced Drudeabsorption does not play an essential role in enhanced residual thermalresponse. This is also confirmed by our estimation of the laser-inducedsurface temperature using the following formula:

${T_{S}(t)} = {{\frac{A\sqrt{a}}{k\sqrt{\pi}}{\int_{0}^{t}{\frac{I( {t - \theta} )}{\sqrt{\theta}}\ {\theta}}}} + T_{0}}$

where A is the absorptance, a is the thermal diffusivity, I is theintensity of incident laser light, k is the thermal conductivity, t isthe time, T₀ is the initial temperature, and θ is the integrationvariable. FIG. 11( a) shows the computed T_(S)(t) induced by the Nd:YAGlaser pulse at F_(abl)=F_(pl)=1.4 J/cm² in 1-atm air and atF_(abl)≈F_(pl)=2.7 J/cm² in vacuum with A=0.25, a=1.0×10⁻⁴ m²/s, k=240Js⁻¹m⁻¹° C.⁻¹, and T₀=20° C. One can see that the maximum surfacetemperature is about 500° C. in air and 1000° C. in vacuum. Theestimated surface temperature in air is below both the melting (660° C.)and boiling (2495° C.) points of Al. The computed T_(S)(t) for rubylaser at F_(abl)≈F_(pl)=1.1 J/cm² in 1-atm air and at F_(abl)≈F_(pl)=2.1J/cm² in vacuum with A=0.28 are shown in FIG. 11( b). Similar to theresults of Nd:YAG laser in FIG. 5( a), the estimated surface temperaturefor ruby laser irradiation in air is also below both the melting andboiling points of Al. Thus when the enhanced thermal coupling occurs in1-atm air, the estimated surface temperature induced by both Nd:YAG andruby lasers is too low to induce a significant increase in absorptance.

The similar general behavior of REC for Nd:YAG (λ=1.06 μm) and ruby(λ=0.69 μm) lasers shows that laser wavelength is relatively unimportantin the visible and near infrared spectral region. Nevertheless, ourexperiment clearly demonstrates that REC of the aluminum sample dependsmainly on laser fluence and ambient gas pressure following ns-laserablation.

The dependence of REC on laser fluence for Al following fs-laserablation in 1-atm air and in vacuum (P=0.01 torr) are plotted in FIG.12. The residual thermal energy coupling is enhanced in air above acertain threshold value of laser fluence, while in vacuum it is reduced.The values of F_(abl), F_(pl), and F_(enh) in air are found to be 0.053J/cm², 0.086 J/cm², and 0.5 J/cm², respectively. These thresholds arewell separated and the enhancement threshold is above the plasmathreshold; i.e. F_(enh)>F_(pl)>F_(abl), in contrast to the ns-laserablation where F_(enh)≈F_(pl)≈F_(abl). We note that our measured valueof F_(pl) in 1-atm air agrees with reported values for Al thin filmdeposited on a silicon substrate. The values of F_(abl) and F_(pl) invacuum are determined to be 0.058 J/cm² and 0.096 J/cm², respectively(see FIG. 12). Contrary to conventional understanding that the residualthermal energy is negligible in an ablated sample following femtosecondlaser ablation, our data show that REC reaches a value of 0.7 indicatingthat, at the highest laser fluence achievable in our experiment (F≈4J/cm²), about 70% of the incident laser energy can be retained in thesample following single-pulse fs-laser ablation in 1-atm air

FIG. 13( a) shows an SEM image of an undamaged surface that ismechanically polished. A view of the sample surface after irradiation inair at F=F_(abl) is shown in FIG. 13( b). (FIG. 13( b) does not show thesame spot on the sample as in FIG. 13( a)). FIG. 13( b) shows thatsurface defects are preferential spots for initial ablation with somesparsely distributed small spherical nanoparticles in the irradiatedarea. FIG. 14 shows a typical laser-induced surface morphology followingablation at F=F_(pl) in 1-atm air. It is seen that surface modificationsare still localized around surface defects, but both the number and thesize of nanoparticles are greater than those at F=F_(abl). Therefore,material ejection in fs-laser ablation appears to be initiated atsurface defects. Open-shutter photographs of the femtosecondlaser-induced plumes taken at F=1.16 J/cm² (higher than F_(pl)) areshown in FIG. 15. The figure shows that the size of the plume in air islarger than that in vacuum (P=0.01 torr).

There are three basic distinctions between ns- and fs-laser ablation.First, fs-laser pulses do not interact with ejected material becausehydrodynamic expansion of ablated material from the irradiated areaoccurs on a timescale much longer than femtosecond pulse duration.Secondly, laser-supported absorption waves that are commonly generatedin ns-laser ablation in a gas medium do not exist in fs-laser ablation.Thirdly, a material irradiated with an intense fs-laser pulse can beheated to a solid-density plasma state.

Example 3

Unique properties of nanomaterials have been extensively studied in thepast and various nanostructures have found numerous applications inoptics including enhanced x-ray emission and enhanced absorption inintense light-matter interaction, and optical biosensing, to name a few.Direct surface nanostructuring (i.e., not from ablated plume deposition)may be used in a number of technological applications, for example,manipulation of optical properties of solids, catalysts, dentalimplants, etc. We performed a detailed study of the morphology ofsurface nanomodifications produced on bulk metals using a femtosecondlaser ablation technique embodied herein. The effects of laser fluenceand number of applied pulses on the generated surface nanostructureswere studied with a scanning electron microscope (SEM). According to anaspect, a set of optimal laser irradiation conditions for metal surfacenanostructuring is disclosed.

In our experiment, we used an amplified Ti:sapphire laser system thatconsisted of a mode-locked oscillator and a two-stage amplifierincluding a regenerative amplifier and a two-pass power amplifier. Thelaser system produces 65-fs pulses with energy around 1 mJ/pulse at a 1kHz repetition rate with a central wavelength λ=800 nm. To produceablation, the laser beam is focused normally onto a bulk sample mountedvertically. To measure the incident pulse energy, a certain fraction ofthe incident light is split off by a beam splitter and measured with apyroelectric joulemeter. The number of laser shots, N, applied to thesample is controlled using an electromechanical shutter. All experimentswere performed in air under atmospheric pressure. The morphology offemtosecond laser-induced surface modifications was studied using a SEM.The studied samples were mechanically polished copper, gold, andplatinum. The range of laser fluence used in the ablation was between0.084 and 9.6 J/cm². The number of applied pulses was varied from 1 to5×10⁴ shots. The ablation threshold was determined as the minimumfluence to generate a surface damage seen under the SEM.

A SEM picture of a copper sample surface prior to laser irradiation isshown in FIG. 16( a). For reference, the ablation threshold for a coppersample was determined to be F_(abl)=0.084 J/cm² following a total ofN=100 shots. The morphology of the irradiated surface was studiedfollowing ablation with laser fluence of F=0.084, 0.16, 0.35, 1.52, 3.7,and 9.6 J/cm² and the number of applied pulses in the range of 1−5×10⁴.A number of representative surface structures produced on the coppersample are shown in FIGS. 16-18. An analysis of the SEM data shows thatthe morphology of femtosecond laser-induced surface nanostructuresdepends both on laser fluence and the number of applied pulses. Theeffect of the total number of shots on nanostructuring at F=0.35 and1.52 J/cm² is shown in FIGS. 16 and 17, respectively. FIG. 16( b) showsthat nanostructures begin to occur on some random localized sites afterone shot at F=0.35 J/cm². A few larger-size structural features are alsoobserved in the central part of the ablated area, as seen in FIG. 16(b). These larger structures may be associated with surface defectsand/or laser beam intensity inhomogeneities. FIG. 16( c) shows ananoscale surface structure produced by two-shot ablation. The structurecomprises both larger nanocavities and nanoprotrusions with sphericaltips of diameter up to about 75 nm. Therefore, the one additional shottransforms the sparsely distributed nanoscale features in FIG. 16( b) tothe cellular-like structures in FIG. 16( c). The surface morphologyafter ablation with 1000 pulses is shown in FIG. 16( d). One can seethat the mean size of nanoprotrusions becomes larger while at the sametime some nanocavities develop into microcavities. The evolution of thesurface structures following ablation at F=1.52 J/cm² and various N isshown in FIG. 17( a-d). At this middle fluence, pure nanostructures areonly generated by ablation with one or two laser shots (FIGS. 17( a) and17(b)). As shown in FIG. 17( c), 10 shot ablation produces both randomnano- and micro-structures. With further increasing N, the proportion ofnanostructures decreases as can be seen in FIG. 17( d), where microscalestructures become dominant. At the highest fluence used in ourexperiment, nanostructures are not present over most of the irradiatedarea and a dominant morphological feature is microroughness. However,nanostructuring can still be observed on the periphery of the ablatedspot where the Gaussian beam intensity is low enough for nanostructuralformation. An example of these surface structural modifications is shownin FIG. 18 for two-shot ablation at F=9.6 J/cm².

The effect of laser fluence on surface structuring can be seen fromanalyzing the surface modifications produced at various F and fixed N asshown for example in FIG. 16( c) (F=0.35 J/cm², N=2), FIG. 17( b)(F=1.52 J/cm², N=2) and FIG. 18 (F=9.6 J/cm², N=2). These images showthat ablation with high laser fluence does not actually inducenanostructures and therefore there exist optimal laser ablationconditions for surface nanostructuring. In order to determine theoptimal conditions for nanostructuring, we performed an SEM study oflaser-induced surface modifications following ablation with a largevariety of F and N. The obtained data are summarized in FIG. 19. One cansee that the most favorable conditions for pure nanostructuring areablation at low and medium values of laser fluence (F<1.5 J/cm²). FIG.19 also shows the range of laser irradiation parameters wherefemtosecond laser ablation produces different combinations of surfacenano-, micro-, and macro-structures.

To determine the mechanism of nanostructuring, we performed a SEM studyon the origin of nanoscale modifications. A representative example ofnascent nanostructures following ablation with F=0.35 J/cm² and N=1 isshown in FIG. 20( b), where the characteristic types of initialnanostructures are labeled. For comparison, FIG. 20( a) shows anundamaged area of the sample using the same scale as in FIG. 20( b). Itis seen in FIG. 20( b) that surface structuring is initiated on random,highly-localized nanoscale sites. The typical structures includecircular nanopores with a diameter in the range of 40-100 nm,randomly-oriented nanoprotrusions with a diameter in the range of 20-70nm and a length of 20-80 nm, nanocavities of arbitrary form, andnanorims around nanocavities. Under these femtosecond laser processingconditions, nanoscale features down to a size of 20 nm are produced. Onecan see from FIG. 20( b) that a nanopore or nanocavity is alwaysimmediately accompanied by a nanorim or nanoprotrusion, indicating ananoscale material relocation to an adjacent site. These one-to-onenanoscale dips and protrusions occur randomly over the laser spot,suggesting an initial non-uniform laser energy deposition. Possiblefactors responsible for the spatial variation of the absorbed laserenergy include: (1) the spatial inhomogeneity of the incident beam; (2)the enhancement of absorption by surface defects; (3) interference ofthe incident laser light with the excited surface electromagnetic wavesdue to structural defects. When the incident laser fluence is close tothe laser ablation threshold, the spatial variations in deposited laserenergy can produce a melt at localized nanoscale sites within theirradiated spot. Once the localized nanoscale melts have been formed, ahigh radial temperature gradient in a nanomelt can induce a radialsurface tension gradient that expels the liquid to the periphery of thenanomelt. This will lead to the formation of nanocavities,nanoprotrusions, and nanorims due to fast freezing of the expelledliquid on the boundary with the solid state material (see FIG. 20( b)).This mechanism may also be used to explain the formation of nanobumps ona thin metal film. These initially induced surface random nanostructurescan enhance the absorption of laser light and facilitate the furthergrowth of surface nanoroughness due to the increased spatial nonuniformenergy absorption. When laser fluence is sufficiently high to produceablation, the atoms ejected from the nanomelts produce a recoil pressurethat squirts liquid metal outside of the nanomelt. For multi-pulseablation, the repeating vaporization and re-deposition of nanoparticlesback onto the surface may also affect the surface nanostructuring. SEMmorphology study at high fluence (F>5 J/cm²; i.e., strong ablation)shows that melt occurs over a large area of the ablated spot (see FIG.18) and the flow dynamics in this large melt pool predominantly resultsin microstructuring. We have also studied the ambient gas pressureeffect on nanostructuring by taking SEM images of platinum followingsingle-pulse ablation in 1-atm air and in a vacuum at a base pressure of8×10⁻³ Torr. Although we have observed a greater amount of re-depositednanoparticles in air than in vacuum, the morphology of nanostructures isstill quite similar under different air pressures. Our study wasperformed with samples mounted vertically. It should be noted that theamount of re-deposited ablated particles back onto the sample surfacemay be different when the sample is positioned vertically versushorizontally, but further studies are required in this aspect ofnanostructuring using fs laser pulses.

Example 4

Laser-induced periodic surface structures (LIPSS) on solids have beenstudied in a number of works in the past. Typically, LIPSS show regulargroove structure with a period on the incident laser wavelength scaleand oriented perpendicularly to the polarization of the incident light.LIPSS are commonly seen following long pulse irradiation on a variety ofmaterials, including semiconductors, metals, and dielectrics.

In contrast to previous work performed mostly at relatively highfluence, we studied the formation of LIPSS on platinum and gold in aspecial fluence regime, namely, at near damage-threshold fluence. Wefound a unique type of LIPS S entirely covered with nanostructures. Adistinctive feature of the nanostructure-covered LIPSS (NC-LIPSS) isthat its period is appreciably less than that of the regular LIPSS whoseperiod is approximately equal to the laser wavelength at normal incidentlaser light. The reduced period of the nanostructure-covered LIPSS iscaused by a significant increase of the real part of the effectiverefractive index of the air-metal interface when nanostructures developon a metal surface that affects the propagation of excited surfaceplasmon polaritons. Nanostructure-covered LIPSS has a variety ofpotential applications, such as modifying optical properties ofmaterials and chemical catalysts where high surface-to-volume ratio is acrucial factor.

In this experiment, we used an amplified Ti:sapphire laser system thatgenerates 65-fs laser pulses with energy about 1 mJ/pulse at a 1 kHzrepetition rate and with a central wavelength λ=0.8 μm. Thehorizontally-polarized laser beam is focused onto a vertically standingmetal sample in air at normal incidence. The number of laser shots, N,applied to the sample is selected with an electromechanical shutter. Westudied the evolution of NC-LIPSS on metals following irradiation withN=1, 2, 4, 8, 10, 20, 30, 40, 50, 100, 200, 300, 400, 500 pulses at neardamage-threshold fluence. The studied metals were platinum and gold. Thelaser fluence of the incident light was varied by changing the distancebetween the focusing lens and sample. To measure the laser pulse energyincident upon the sample, a fraction of the incident laser beam is splitoff by a beamsplitter and diverted to a pyroelectric joulemeter. Themorphology of the produced periodic structures is examined using ascanning electron microscope (SEM). The surface profile is measured withan atomic force microscope (AFM). All sample surfaces were mechanicallypolished using 0.1 μm grade aluminum oxide powder.

The evolution of surface structures produced on Pt following ablation atnear damage-threshold laser fluence of F=0.16 J/cm² is shown in FIGS.21( a)-(b). FIG. 21( a) demonstrates surface random nanoroughnessproduced after 10 shot ablation. The inset in FIG. 21( a) shows thatthis initial surface modification is characterized by nanocavities andnanoprotrusions of various forms. At N=20, a microscale periodic patternstarts to form over the initially produced random nanoroughness. At thisstage, only small patches of periodical structures are observed invarious isolated locations within the irradiated spot, referred to belowas intermediate LIPSS. With increasing N, the intermediate LIPSS growand coalesce into a clear extended LIPSS with a period of 0.61 μm atN=30 (FIG. 21( b). For N greater than 70 shots, LIPS S starts todisappear gradually in the central spot area (FIGS. 22( a) and 22(b)).However, clear NC-LIPSS continue to form in the peripheral area (FIGS.22( c) and 22(d). Using AFM, the initial undamaged surface rms roughnessis found to be about 5.6 nm after polishing, and a typical AFM surfaceprofile measurement on Pt is shown in FIG. 23( a). Following ablationwith N=1, 2, and 10 shots, surface rms roughness is found to be about16.5, 35.2, and 79.8 nm, respectively. FIG. 23( b) shows typical surfaceroughness after N=10 shots. The surface profile of LIPSS after 30 lasershots is shown in FIG. 24. To gain the insight of how initialnanoroughness affects the formation of NC-LIPSS, we performed a SEMstudy of the formation of NC-LIPSS with samples of different initialsurface conditions. We found that the extended LIPSS is produced with asmaller number of laser shots when the sample has a greater surfacenanoroughness. To understand the material dependency in forming thenanostructure-covered LIPSS, we also performed a detailed SEM study ofsurface structural modifications on Au. Our data show that the generaltrend is similar for Au and Pt in forming the initial nanoroughness. Theperiod of nanostructure-covered LIPSS on Au is observed to be 0.58 μmand is also markedly less than the laser wavelength (FIG. 25). However,the periodic patterns on Au are much less clear compared to Pt.Recently, we performed a comparison study on regular LIPSS on variousmetals following femtosecond laser radiation where LIPSS showsdistinctly different level of morphological clearness among variousmetals even under identical experimental conditions. The electron-phononenergy coupling coefficient, g, is shown to directly correlate to themorphological clearness of LIPSS. A larger g coefficient usually leadsto more pronounced LIPSS. In this study, g coefficient for Pt and Au are25×10¹⁶ and 2.1×10¹⁶ W/m³K, respectively, and the much larger gcoefficient explains why LIPSS is much more clear on Pt than Au.

The periodic patterns induced by femtosecond laser processing aredistinctly different from those produced by longer pulses in twoaspects. First, femtosecond laser-induced periodic structures arecovered by random nanostructures. Secondly, the LIPSS period induced byfemtosecond pulses at normal incidence is appreciably less than thelaser wavelength while the period is roughly equal to the wavelength forlonger pulses. To account for our observation, we carefully examined theevolution of surface structural modifications on both Pt and Au, and wepropose the following mechanism for the formation of NC-LIPSS. The firstfew laser shots usually produce sparsely and randomly distributednanostructures. It is known that surface plasmons, both localized andpropagating along a surface, can be excited by coupling laser energyinto nanostructures. With further increase of the number of laser shots,more nanostructures appear allowing excitation of more localized andpropagating surface plasmons. The produced nanoroughness includesnanorods, nanocones, and nanospheres, and these nanostructures willexcite propagating cylindrical surface plasmons that subsequentlyinterfere with the incident light. This interference causes theformation of intermediate periodic surface microstructures. As thenumber of laser shots increases, the intermediate microstructures willgrow as well as the area occupied by these structures. The developedintermediate periodic surface microstructures will further excitepropagating plane surface plasmons that interfere with the planeincident laser light wave, and this interference will finally result inthe permanent extended periodic microstructures.

For normally incident linearly polarized light, the period d of thesurface grating formed due to the interference between the incidentlaser light and the excited surface plasmon wave is given by equation 1as:

d=λ/η  (1)

with g|E, where λ is the incident light wavelength, η=Re[∈/(∈+1)]^(1/2)is the real part of the effective refractive index of the air-metalinterface for surface plasmons, ∈ is the dielectric constant of themetal, g is the grating vector, and E is the electrical field vector ofthe incident wave. For a plane vacuum-metal interface, η is calculatedto be 1.0096 at λ=800 nm for Pt (∈₁=−15.5 and ∈₂=23.5) and 1.022 for Au(∈₁=−23.4, ∈₂=1.55). Using Eq. (1), the grating period is found to be0.79 μm for Pt and 0.78 μm for Au. However, the observed period is 0.61μm for Pt (FIG. 1 (b)) and 0.58 μm for Au (FIG. 5). If we substitutethese values of the observed period into Eq. (1), we will have η=1.31for Pt and η=1.38 for Au. To explain this discrepancy, we note that thetable values of ∈₁ and ∈₂ for Pt and Au are obtained from smooth surfaceand at room temperature, and therefore these values may not be suitablewhen the metals are heated by high-intensity femtosecond pulses andcovered with nano- and micro-structures. To better understand thehigh-intensity effects on NC-LIPSS period, we performed a detailed studyof LIPSS in various locations within the damaged spots on metals. Fromthese data summarized in Table 1, we can see that the NC-LIPSS periodremains the same in the central and peripheral areas of an irradiatedspot despite the fact that the two locations have different intensitiesdue to the Gaussian beam profile. On the other hand, the period of ourNC-LIPSS decreases with increasing N when the surface roughness growswhile the light intensity remains constant. Furthermore, the NC-LIPSSproduced using a higher fluence of 0.16 J/cm² exhibit a similar periodas that produced at F=0.084 J/cm². Our observations indicate that thehigh-intensity effect on dielectric constant is not essential, whereasthe effects of surface morphology (nano- and micro-roughness) are moredominant. It is known that surface roughness causes an increase in themodulus of the surface plasmon wave vector, and this will correspond toan increase in the real part of the refractive index. According to Eq.(1), an increased real part of the refractive index for propagatingsurface plasmons will cause a reduced NC-LIPSS period, which agrees withour experimental observation.

TABLE 1 Nanostructure-covered LIPSS period in different areas of theirradiated spot on platinum at F = 0.084 J/cm². LIPSS period (μm) Numberof shots Central area Peripheral area 30 0.62 0.61 50 0.58 0.61 100 0.570.57 200 0.55 0.54 500 0.55 0.53

Under certain conditions we also produced a large number ofnanoprotrusions and nanocavities on a metal surface (see FIG. 22). Thenanostructures produced can greatly increase the effective surface area,which may be of importance in many technological applications, such asbut not limited to producing better chemical catalysts where a highsurface-to-volume ratio is a crucial factor.

Example 5

In this study we performed a femtosecond laser surface treatment oftitanium to help determine the potential of this technology for surfacestructuring of titanium implants. We found find that femtosecond laserprocessing produces a large variety of nanostructures (nanopores,nanoprotrusions) with a size down to 20 nm, multiple parallel groovedsurface patterns with a period on the sub-micron level, microroughnessin the range of 1-15 μm with various configurations, smooth surface withsmooth micro-inhomogeneities, and smooth surface with sphere-likenanostructures down to 10 nm. Also, we have determined the optimalconditions for producing these surface structural modifications.Femtosecond laser treatment may produce a richer variety of surfacestructures on titanium for implants and other biomedical applicationsthan long-pulse laser treatments.

Due to good biostability, biocompatibility, mechanical performance, andlong-term durability, titanium has been widely used in a variety ofbiomedical applications such as dental and orthopedic implants, andimplantable electronic devices. In numerous in vitro and in vivostudies, surface topography of titanium implants has been shown to beimportant in enhancing implant performance. It has been shown that bothmicrostructures and nanostructures influence biological processes atimplant interfaces. Various methods of implant surface structuring havebeen studied in the past such as grit-blasting, chemical etching, lasertreatment, and the combinations of the various methods. Recent studieshave shown that laser processing of implant surfaces provides bothsuitable surface topography and less surface contamination as comparedwith other methods. Another advantage of laser processing is that thetechnique is also suitable for texturing of implants of more complicatedshapes. In the past, surface structures have been produced usinglong-pulse lasers, including nanosecond Nd:YAG laser, copper vaporlaser, nanosecond excimer lasers, picosecond Nd:YAG laser, andsub-picosecond excimer laser. Femtosecond lasers have advantages overnanosecond lasers in aspects of higher precision, reduced heat-affectedzone, and smaller amount of debris around the ablated spot.

The effects of laser fluence and the number of applied pulses onlaser-induced surface topography in titanium are reported herein. Wefound that a femtosecond laser produces a large variety ofnanostructures (nanopores, nanoprotrusions) with a size down to 20 nm,multiple parallel grooved surface patterns with a period on thesub-micron level, microroughness in the range of 1-15 μm with variousconfigurations, smooth surface with smooth micro-inhomogeneities, andsmooth surface with sphere-like nanostructures down to 10 nm. Ourresults suggest that femtosecond laser treatment can produce a richervariety of surface structures on titanium for implants and otherbiomedical applications than long-pulse laser treatments.

Commercially pure titanium flat plates with a dimension of 15×17×1.5 mmwere used in our experiment. The plates were mechanically polished using0.1-μm-grade aluminum oxide powder and further cleaned with acetone. Forsurface texturing, we used an amplified Ti:sapphire laser system thatgenerates 65-fs laser pulses with the pulse energy over 1 mJ at a 1 kHzrepetition rate with a central wavelength of 0.8 μm. The laser beam ishorizontally polarized and was focused at normal incidence onto avertically standing titanium sample in air at a pressure of 1 atm. Forlaser beam focusing, we used an achromatic lens with a focal length of20 cm. The laser fluence of the incident light was varied by changingthe distance between the focusing lens and the sample. The diameter oflaser-irradiated spots on the titanium sample was varied from 100 to1200 μm. The number of laser shots, N, applied to the sample wasselected with an electromechanical shutter. The surface structuring oftitanium was studied following the treatment with laser fluence ofF=0.067, 0.084, 0.16, 0.35, 0.48, and 2.9 J/cm² and the number ofapplied pulses, N, in the range of 1-30,000. Following femtosecond lasertreatment, the topography of surface modifications was studied using aSEM.

As a reference, FIG. 26( a) shows a SEM image of the titanium surfaceprior to laser irradiation. FIGS. 26( b) through 26(d) demonstratesurface topography produced by femtosecond laser processing atnear-damage-threshold fluence of F=0.067 J/cm² for different numbers oflaser shots, where the characteristic features are random nanopores andsphere-like nanoprotrusions with the size down to about 15-20 nm.Laser-induced surface nano-topography depends on both the number ofapplied pulses and laser fluence. At higher fluence of F=0.084 J/cm²,the nanoroughness produced is shown in FIGS. 27( a) through 27(d), wherethe average size of the nanostructures at this higher fluence is largerthan those at lower fluence in FIG. 1. For N>10-15, periodic ordering ofsurface nanoroughness begins to occur. FIGS. 28 and 29 show some typicalperiodic patterns for laser fluences F=0.067 and 0.084 J/cm²,respectively. The period of the grooves is about 0.53 μm. These periodicpatterns with sub-micron periods are covered with nanoroughness, asshown in detail in FIGS. 28( d) and 29(d). With increasing laserfluence, the periodic patterns are less likely produced andmicroroughness becomes a more dominant surface structure. FIG. 30 showssurface topography produced following treatment at F=0.16 J/cm² atvarious N. At this middle-level laser fluence, pure nanoroughness isobserved only after one-shot laser processing (FIG. 30( a)). A clearmicroscale roughness covered with nanoroughness develops after 20-shottreatment (FIG. 30( b)). With further increasing N, microroughnesscontinues to develop with deepening of cavities (FIG. 30( c)). At alarge enough N, columnar surface micro-structures covered withnanoroughness are seen in FIG. 30( d). At higher laser fluence of F=0.35J/cm², a combination of nano- and micro-structures is produced afteronly one laser shot, as shown in FIG. 31( a). With increasing N,columnar microstructures rapidly develop as the dominating structures(see FIGS. 31( b)-31(d)). When the laser fluence is increased to thelevel of F=0.48 J/cm², a different type of surface microstructures isobserved, as shown in FIGS. 32( a)-32(c). At this laser fluence and forN>1000, a pore of the size of the focused laser beam can be created. Anexample of such a pore with the diameter of 350 μm is shown in FIG. 32(d), where microstructures are also seen at bottom of the pore. At thehighest fluence used in our experiment, F=2.9 J/cm², one laser shot canproduce surface melting over the entire irradiated surface area, andresolidification of this surface melt results in a smooth surfacecovered with some micro-inhomogeneities as shown in FIG. 33( a).Following two-pulse irradiation, an even smoother surface is seen inFIG. 33( b). A magnified picture showing nanoscale features of suchsmooth surfaces is shown in FIG. 33( c). A detail picture of thetitanium surface after four laser shots is shown in FIG. 33( d), whereone can see nanoscale structures as small as down to 10 nm. The smoothsurface is produced with a low number of laser shots (N<10). At a largerN (N>10), micro-inhomogeneities develop rapidly and eventually a craterof the size of the focused laser beam will be formed.

It has been shown that implant surface topography is an important factoraffecting the behaviors of both proteins and cells on implant surfaces.It is generally accepted that proteins typically respond to surfacestructural features (pits, pillars, steps) about 1-10 nm, while cellscan be sensitive to structural features on the scale of 15 nm-100 μm. Itwas also found that structured implants have a better mechanicalinterlocking of the bone-implant interfaces than smooth implant surfacesdue to an increased surface area. Also, it has been reported thatextended parallel groove structures may cause cells to align and migratealong the grooves, a contact guidance phenomenon. Our SEM study showsthat all of these types of surface textures can be produced byfemtosecond laser treatment.

Little work has been done on laser fabrication of surface nanostructureson titanium. Our study shows that femtosecond laser technique canproduce a large variety of both pure nanostructures (FIGS. 26( b)-26(d),27(a)-27(d), and 33(c)) and various combinations of micro- andnanostructures (FIGS. 28( d), 29(d), 30, 31). There are two types ofpure nanostructures observed in our experiment. The first type (FIGS.26( b)-26(d), 27(a)-27(d)) is produced at low laser fluence (near thedamage threshold) and a low number of laser shots; the size of thesenanostructures is down to 20 nm. The second type (FIGS. 33( c) and33(d)) is produced at high fluence and low N when laser irradiationcauses the surface to melt uniformly over the entire irradiated area;the size of these nanostructures is down to 10 nm.

Examination of shot-to-shot SEM images of surface topography suggeststhe following mechanism for the formation of nanostructures of the firsttype. It is seen from FIG. 26( d) that a nanopore is always accompaniedby a nearby nanoprotrusion, indicating a nanoscale material relocationto an adjacent site. This one-to-one nanoscale pores/protrusionsrelationship occurs randomly over the laser spot, suggesting an initialnon-uniform laser energy deposition. When the incident laser fluence isclose to the laser damage threshold, spatial nonuniformity in thedeposited laser energy can produce a melt at localized nanoscale siteswithin the irradiated spot. Once the localized nanoscale melt has beenformed, a high radial temperature gradient in a nanomelt can induce aradial surface tension gradient that expels the liquid to the peripheryof the nanomelt. This can lead to the formation of nanocavities andnanoprotrusions due to fast freezing of the expelled liquid on theboundary with the solid state material. These initially induced surfacerandom nanostructures enhance the absorption of laser light andfacilitate further growth of surface nanoroughness with increasingnumber of laser shots due to the increased spatial non-uniform energyabsorption. When laser fluence is sufficiently high to produce ablation,particles will be ejected from the nanomelts and produce a recoilpressure that squirts the liquid metal outside of the nanomelt. Itshould be noted that for multi-pulse ablation, the repeatingvaporization and re-deposition of nanoparticles back onto the surfacecan also promote surface nanostructuring. As seen from FIGS. 26 and 27,the average size and density of femtosecond laser-induced nanostructuralfeatures can be controlled by varying both the laser fluence and numberof laser shots.

Mechanisms for the formation of nanostructures of the second type cannotbe straightforwardly derived from our SEM study. The formation of thesenanostructural features may be due to redeposition of ablatednanoparticles back onto the irradiated surface.

Multiple parallel grooved surface patterns for biomedical applicationsare commonly produced using lithographic or laser holographictechniques. However, fabrication of these type of patterns onbiomaterials using a single laser beam has not been reported. Below wediscuss the optimal conditions for producing these structures andexplain the physical mechanisms of their formation.

Our study shows that optimal conditions for producing periodic groovepatterns on titanium are at near-damage-threshold fluence and with thelaser shot number in the range between 20 and 800. In the past, multipleparallel grooved surface patterns have been produced by long-pulselasers and are known as laser-induced periodic surface structures(LIPSS). The formation of LIPSS on metals is believed to result from theinterference of the incident laser light with the excited surfaceplasmon polaritons that result in spatial periodic energy distributionon the surface. Usually, LIPSS shows a regular groove structure with aperiod on the incident laser wavelength scale and is orientedperpendicularly to the polarization of the incident light. Our resultsof the evolution of surface structural modifications on titanium suggestthe following mechanism for the formation of the observed LIPSS. In ourexperiment, the first few laser shots produce sparsely and randomlydistributed nanostructures. It is known that propagating cylindricalsurface plasmons can be excited by coupling laser energy intonanoroughness, and this can give rise to their interference with theincident light. This interference will, first, cause the formation ofintermediate periodic surface structures in localized areas of theirradiated spot. An example of such intermediate periodic surfacestructure can be seen in FIG. 26( c). With further increasing number oflaser shots, the number of intermediate periodic structures will grow aswell as the area occupied by these structures. The developedintermediate periodic surface structures will further excite propagatingplane surface plasmons and their interference with the plane incidentlaser light wave will, finally, result in the permanent extendedperiodic grating.

For linearly polarized incident laser light, the period d of the surfacegrating formed due to the interference between the incident laser lightwave and the excited surface plasmon wave is given by d=λ/(η±sin θ) withg|E, where in this equation λ is the incident light wavelength,η=Re[∈/(∈+1)^(1/2) is the real part of the effective refractive index ofthe air-metal interface for surface plasmons, E is the dielectricconstant of the metal, θ is the laser light incidence angle, g is thegrating vector, and E is the electrical field vector of the incidentwave. The above equation shows that the period of laser-fabricatedgrating can be varied by changing the laser wavelength, the incidenceangle, or the real part of the effective refractive index. An importantparameter affecting the cell behavior is known to be groove depth, andthis parameter in fabricating LIPSS can be controlled by the number ofapplied laser shots.

A unique feature of the periodic groove structures produced according tothe embodiments herein is that both ridges and grooves are covered withnanoroughness following femtosecond laser treatment, in contrast torectangular surface grooves fabricated using lithography techniques thatusually have smooth ridges and rough floors.

Laser microtexturing of titanium has been studied in the past usinglong-pulse lasers. It has been shown that laser processing of implantsurfaces provides both suitable surface microstructures and the leastsurface contamination as compared with other methods. As shown in FIGS.30( b)-30(d), 31(a)-31(d), 32(a)-32(c), and 33(a), a rich variety ofmicrostructures can be produced by femtosecond laser treatment, andthese structures can be characterized as the following two types. Thefirst type (see FIGS. 30( b)-30(d), 31(a)-31(d), 32(a)-32(c)) isproduced at the middle levels of the laser fluence (F=0.16 and 0.35J/cm²). The characteristic size of this type of microroughness is in therange of 1-15 μm. Both the characteristic size and configuration of thesurface microroughness can be controlled by both laser fluence and thenumber of applied shots. This type of microroughness seems to be onlyproduced by femtosecond laser treatment. The second type ofmicroroughness (see FIG. 33( a)) is characterized by a smooth surfacewith smooth micro-inhomogeneities. This type of microroughness isproduced at the highest laser fluence in our experiment (F=2.9 J/cm²)when melting occurs over the entire irradiated area. If the meltedsurface has some structural inhomogeneities, fast resolidification ofthis melted surface may result in smooth micro-scale roughness.

Some parts of implant surfaces may be required to be smooth. Previously,nanosecond excimer lasers have been used for polishing machined titaniumimplants, and effects of both polishing and cleaning of the surfaceshave been reported). Our study shows that smoothed surface can be alsoobtained with femtosecond laser treatment, as shown in FIG. 33( b).

It is known that open pores with a diameter in the range of 100-400 μmcan improve the strength of bone-implant interfaces. Recently,long-pulse lasers have been used for fabricating 100-300 μm pores onTi6Al4V implants. Our study shows that pores of this size can beproduced with femtosecond laser treatment, as shown in FIG. 32( d). Wenote that we can further produce various surface structures on the porebottom through femtosecond laser treatment.

Example 6

Another application of femtosecond laser surface structuring to producethe materials processing of the presently embodied invention is toprovide the controllable modification of the optical properties ofmetals, where these optical properties range from the UV to THz spectralrange, and where the modifications may be used to create various black,grayed, and colored metals.

As an example, FIG. 34 shows the % reflectance from 0.25 to 2.5 μm of“black” aluminum obtained by the materials processing methods of thepresent invention. In the visible this aluminum appears pitch black asillustrated in FIG. 35( a).

By varying the materials processing parameters, we have also producedaluminum that appears to be various shades of gray. Thus in the case ofthe grayed aluminum as shown in FIG. 35( b), the materials processingwas performed at laser fluence F=7.9 J/cm², a scanning speed of thelaser beam across the surface of the Al of v=1 mm/s, and translationbetween scanning lines S=100 μm. The two gray shades of aluminum shownin FIG. 35( b) are obtained by varying the laser pulse repetition rate(f=100 Hz for the darker shade and 93 Hz for the lighter one). Thespectral reflectance of this darker gray aluminum sample is shown inFIG. 34.

In addition to producing various shades of gray as discussed above, thematerials processing methods of the present invention can also producecolored metals; i.e., metals that appear to have a particular color orthat appear to have multiple colors.

To produce colored metals, two types of femtosecond laser processingtechniques were performed. The first technique involved tailoringlaser-induced surface random structures, while the second techniqueproduced femtosecond laser-induced periodic surface structures (FLIPSS).The colored metals produced by the first technique exhibit the sameapparent color at various viewing angles, while the colored metalsproduced by the second technique exhibit different colors at differentviewing angles due to a grating effect.

FIG. 35( c) shows a picture of a colored aluminum sample produced by thecontrolled tailoring of random surface roughness. The aluminum appearsgolden in color because the tailored surface structures preferentiallyenhance the absorption at blue and green wavelengths. The spectralreflectance of the golden aluminum is shown in FIG. 34.

Colored metals produced by the second technique, FLIPSS, exhibitdifferent colors at different viewing angles. FIG. 36 shows variouscolors of an aluminum sample structured with FLIPSS under experimentalconditions of F=0.05 J/cm², f=83 Hz, v=1 mm/s, and S=100 μm. Thespectral reflectance of the color aluminum structured with FLIPSS isshown in FIG. 34. Structuring with FLIPSS can cause a polarizationeffect on the absorption of light that provides an additional way forcontrolling the optical properties. The size of the optically modifiedmetal surface area can be as small as a tightly focused laser spot; i.e.down to about 10 μm, or as large as desired by using a scanning laserbeam (for example, FIGS. 35 and 36 show samples with structurallymodified area of about 24 mm in diameter).

Given the additional advantages of laser processing such as lowcontamination and capability to process complicated shapes, the black,grayed, and colored metals created by femtosecond laser surfacestructuring have numerous potential applications in such areasincluding, but not limited to, photonics, plasmonics, optoelectronics,stealth technology, thermal radiation sources, solar cell absorbers,radiative heat transfer devices, infrared sensing, biooptical devices,thermophotovoltaics, and airborne/space borne devices.

While specific embodiments of the present invention have been describedherein, it will be appreciated by those skilled in the art that manyequivalents, modifications, substitutions, and variations may be madethereto without departing from the spirit and scope of the invention asdefined in the appended claims.

1. A surface-structured metal, comprising: a base metal having at leastone of a femtosecond laser pulse-induced nanostructured surface and ananostructure-covered laser-induced periodic surface structure(NC-LIPSS).
 2. The surface-structured metal of claim 1, wherein thenanostructure-covered laser-induced periodic surface structure has aperiod that is less than a period of a non-nanostructure-coveredlaser-induced periodic surface structure (LIPSSs) in the metal surface.3. The surface-structured metal of claim 1, wherein the base metal has apolished surface.
 4. The surface-structured metal of claim 1, whereinthe base metal includes substantially any metal and metal alloy.
 5. Thesurface-structured metal of claim 1, wherein the base metal is one ofaluminum, gold, titanium, copper, platinum, tungsten, silver, titaniumalloy, aluminum alloy, brass, and stainless steel.
 6. Thesurface-structured metal of claim 1, wherein the surface-structuredmetal has a colored appearance.
 7. The surface-structured metal of claim6, wherein the surface-structured metal has at least one of a blackcolor, a blue color, a gold color, a yellow color, a gray color, a redcolor, and a combination thereof.
 8. The surface-structured metal ofclaim 6, wherein the surface-structured metal has an iridescence.
 9. Thesurface-structured metal of claim 1, wherein the base metal is aluminumand the color is black.
 10. The surface-structured metal of claim 1,wherein the base metal is aluminum and the color is gold.
 11. Thesurface-structured metal of claim 1, wherein the base metal is titaniumand the color is blue.
 12. The surface-structured metal of claim 1,wherein the base metal is platinum and the color is gold.
 13. Thesurface-structured metal of claim 6, having a color that changes as afunction of viewing angle.
 14. The surface-structured metal of claim 6,having a color that is substantially constant as a function of viewingangle.
 15. The surface-structured metal of claim 1, having anabsorptance equal to or greater than 0.8 for a wavelength that is equalto or greater than about 200 nanometers.
 16. The surface-structuredmetal of claim 1, having an absorptance equal to or greater than 0.2 fora wavelength that is equal to or greater than about 50 micrometers. 17.The surface-structured metal of claim 1, having an absorptance equal toor greater than 0.9.
 18. The surface-structured metal of claim 1, havingan absorptance equal to or greater than 0.5, wherein the base metal hasan absorptance substantially less than 0.5 at normal incidence.
 19. Thesurface-structured metal of claim 1, wherein the surface structurefurther includes a plurality of microscale aggregates.
 20. Thesurface-structured metal of claim 19, wherein the microscale aggregatesfurther comprise at least one of micropores, microgrooves, andmicrochannels.
 21. The surface-structured metal of claim 19, wherein thesurface structure further includes a periodic plurality of elongategrooves.
 22. The surface-structured metal of claim 1, wherein thesurface structure comprises a plurality of nanobranches.
 23. Thesurface-structured metal of claim 22, wherein the surface structurefurther comprises a plurality of spherical nanoparticles.
 24. Thesurface-structured metal of claim 1, wherein the surface structurecomprises nanostructures having a shape of nanovoids.
 25. Thesurface-structured metal of claim 1, wherein the surface structurecomprises redeposited nanoparticles of ablated base metal.
 26. Thesurface-structured metal of claim 1, wherein the base metal is a bulkmetal.
 27. The surface-structured metal of claim 1, wherein the basemetal is a thin film.
 28. A surface-structured metal having an inducedabsorptance that is equal to or greater than 0.8 for a wavelength thatis equal to or greater than about 200 nanometers.
 29. Thesurface-structured metal of claim 28, comprising an inducednanostructure in a surface of the metal.
 30. The surface-structuredmetal of claim 28, comprising substantially any metal and metal alloy.31. The surface-structured metal of claim 29, wherein the inducednanostructure is a nanostructure-covered laser-induced periodic surfacestructures (NC-LIPSS).
 32. The surface-structured metal of claim 29,wherein the induced nanostructure comprises a plurality of nanobranches.33. The surface-structured metal of claim 32, wherein the inducednanostructure further comprises a plurality of spherical nanoparticles.34. The surface-structured metal of claim 28, wherein the metal is abulk metal.
 35. The surface-structured metal of claim 28, wherein themetal is a thin film.
 36. A method for treating a base metal,comprising: exposing a surface region of the base metal to a femtosecondduration laser pulse having a fluence, F, that is sufficient to alter asurface structure of the base metal; and altering the surface structureof the base metal by creating a nanostructure in the surface.
 37. Themethod of claim 36, comprising exposing the surface region of the basemetal to a single femtosecond duration laser pulse.
 38. The method ofclaim 36, comprising creating a plurality of nanobranch structures. 39.The method of claim 38, further comprising creating a plurality ofspherical nanoparticle structures.
 40. The method of claim 38, furthercomprising creating a plurality of nanovoids.
 41. The method of claim38, further comprising creating surface microstructures.
 42. The methodof claim 36, further comprising creating at least one of micropores,arcuate microgrooves, and central microchannels.
 43. The method of claim36, comprising creating a plurality of nano-covered laser-inducedperiodic surface structures (NC-LIPSSs).
 44. The method of claim 43,further comprising creating a plurality of surface macrostructures. 45.The method of claim 44, comprising scanning a femtosecond durationpulsed laser beam across the surface of the base metal at a selectedvelocity.
 46. The method of claim 45, comprising creating a periodicplurality of grooves.
 47. The method of claim 46, wherein the periodicplurality of grooves has a period equal to a translation step betweentwo adjacent laser scanning lines.
 48. The method of claim 36, furthercomprising ablating material from a region of the surface andredepositing the ablated material on the surface.
 49. The method ofclaim 37, comprising exposing the surface region of the base metal tobetween 1-10 additional femtosecond duration laser pulses.
 50. Themethod of claim 49, comprising exposing the surface region of the basemetal to between 10-100 additional femtosecond duration laser pulses.51. The method of claim 50, comprising exposing the surface region ofthe base metal to between 100-300 additional femtosecond duration laserpulses.
 52. The method of claim 51, comprising exposing the surfaceregion of the base metal to greater than 300 additional femtosecondduration laser pulses.
 53. The method of claim 44, wherein the NC-LIPSShas a period, d, that is less than the laser wavelength, λ.
 54. Themethod of claim 53, comprising controlling the period of the NC-LIPSS bycontrolling at least one of λ, the incidence angle, θ, and the real partof the effective refractive index of an air-metal interface, η,according to the equation d=λ/(η±sin θ).
 55. The method of claim 36,comprising polishing the base metal surface prior to exposing thesurface to the femtosecond duration laser pulse.